90% of most CE strengths & weaknesses are determined by the hypothetical case of two offensive jutsu of the same rank and size colliding with each other head-on from opposite directions. If CE A wins and proceeds towards the opponent, CE A is strong against element B. If element B wins and proceeds, CE A is weak against element B. If they cancel each other out, they are neutral to each other.
When you're facing an opponent, don't be hasty. Research him or her well to know what to expect and prepare for it. Read his or her bios in detail, his or her past fights and specially his or her custom jutsu thread. The more you know, the less likely you are to be caught unprepared. Pay attention to his or her most used techniques as they will reveal his or her overall fighting style and don't forget to read his or her custom weapon carefully.
When you start RPing, it's always better to make your first bio yourself using the standard coding. This way you actually learn to make a bio yourself. Making it fancy and/or go to a bio shop is something you can always do afterwards with future bios and updates. No need to risk your first bio getting declined because you let someone else make it and aren't able to understand how to modify it when a mod doesn't want to approve it because something is wrong.
When you have a free time from training, battles, etc, don't slack! Being a good roleplayer requires dedication and hard work! Read training threads, battles, battle tests, jutsu list or even the cannon submission threads. Read the battle and bio rules and anything you can that is useful like the custom submission threads or even bios. Try to get familiar with the rules and the abilities that exist. Knowledge doesn't fill up any space so stack up on it!
Fighting Style. To completely master the art of battling, you need to be able to a) be aware of your fighting style and b) detect and determine your opponents fighting style. Fighting is all about strategy and you can only create a viable one if you know your own way of fighting and your opponents. Its that knowledge that will guide you to avoid exposing your weakness while still trying to exploit your opponents. For that you need both experience and research.
Don't forget the basics and be rational with the use of your chakra. People often forget the use of the basic Ninjutsu techniques and often rely only on high ranked elemental attacks. However, a kunai, which consumes no chakra, can be deadlier than most elemental techniques...
When making a CE, the explanation behind it is vital. Many people think that just having an idea is enough and that if they stick some stuff together, it's enough to get it approved. This often results in non-sensnical and inconsistent explanations where regularly their creators blow the CEs abilities out of proportion or even make them up just so that they could fit their own needs. You need to realize that even if it's 100% certain something exists in the Narutoverse, it might be completely impossible to ever make a CE out of it if the explanation does not suffice.
Custom Jutsu... Oh yeah, the holy grail of RP status. First thing you need to do is follow the template and the rules posted in the first post of the thread. Almost half the submissions are declined for wrong chakra or damage info or even formatting mistakes. Template is essential and so is following the simple rules of the first page of the CJ thread. Then you need to use something simple: logic. We don't require a scientific explanation with too much detail but you need to establish a common reasoning as to why you achieve what you claim you achieve. Saying it happens "just because" is not going to work. And this includes not overstrecthing. Don't pile up hundreds of effects into a simple technique. Keep it clean, simple and easy. Third step is simply to be smart. Don't do things done before. If its too simple, too generic and can be achieved through a cannon technique, 95% chances it won't be approved. And forth step. Anything you can't find a defense for is by default declined. If you want a technique, make sure its something that can be defended from by others. Even if its hard, it needs to be possible to defend from it without a specific technique or ability for it. And finally, don't make customs "just because you can". Make customs that fill a given purpose and fix a need you felt in battle, not simply "because". A custom without purpose is a voided one by default.
Don't forget you can place additional restrictions on the usage of specific elements, abilities and even chakra usage during a regular fight in the Arena. This way you can make it harder or even impossible for your opponent to use his strongest CJ or CE and make it possible to have fights that have the same limitations as canon fights. As a result both you as your opponent has to think things through much more before acting. Both parties logically have to agree to all the terms of the fight first.
When you have your first custom techniques approved, you're bound to have a plethora of members swarming you with requests to learn them. Don't fall under that temptation easily and light hearted. You can only teach each custom to 6 people throughout your RP career and those cannot be revoked. So you must be sure the trade is beneficial to you and that you won't regret it in the future. Also, remember that customs are, in some circumstances, the main thing that makes each rpear unique so if you spread your "uniqueness" too thin, you'll end up being just one more in the crowd. A Custom Technique is one of the few trade goods in the RP so you must value it and treat it with due respect.
Making long custom histories for your bio(s) is something you do on your own accord. You are allowed to do it it, however then you also need to accept the possibility it might take a while before your bio gets approved and that other bios get approved before you that were posted after yours. The time it would take to check that history, several other bios could have been checked.
When you take a battle test for a special ability or character, you need to remember that its a different fight from any other you've taken: you'll be evaluated on your skill. And not only your skill with the given bio but also your basic skill.
Key aspects you need to make sure you've mastered before attempting a battle test:
If you are comfortable in your skill with these aspects you need to study the abilities you'll be using in the battle test. Read the techniques, simulate their use in your mind, read the bio abilities and read other similar or equal tests, for both passing ones and failed ones. Learn as much as you can.
In the fight itself, the tester will pressure you, so you can show a solid defense, but he will also allow openings so that you can pressure him as well, demonstrating your own abilities. Be sure to seize that opportunity. If you can, take control of the battle flow and put him on defense; that will earn you extra points on grading. Don't go off character with technique/elemental usage unless you have too and never do the same counter or use the same basic technique more than once; you need to show diversity. This said, avoid cliche counters and moves; we have all seen hurricane fist's escape, not a showcase of skill at this point. Finally, don't be afraid to go all out if needed; you'll have a limited amount of turns to show what you can do so try your best to do it, showing both simple, basic skills as well as advanced combos and strategy.
Before trying to explore a KG, a HA or a Special Bio, be sure you have mastered the basic aspects of the RP and that you have a decent amount of training and skills. You'll find that you'll be able to see those abilities in a different manner and it will be easier to learn how to use them if you have some experience beforehand.
Any additional metal, crystal, fire, ice etc. CE are not forbidden to be made, it's just that the chance that they will get approved is extremely small as the difference between the already existing ones and new ones is way to small to consider them as new CE unless it has something that sets it fundamentally apart from already approved/existing elements.
All of the basic shinobi skills have to be learned (for example the 5 elements, genjutsu, taijutsu etc.), no exceptions. In theory everything else also has to be learned, however regularly it happens that in the case of the lesser used side skills (for example Akamichi, Nara, Yamanaka clan abilities, smoke and ink ninjutsu, lava, storm, ice KG etc.) no sensei/sempai/shishou are currently eligible to teach it. In such a case you get the skill by default, however first you need to get a confirmation by an RP mod, preferably by the one that's in charge of training-related stuff. It's also advised that when you are allowed to use such a specific ability without training you apply after a while, when you got used to that ability, at least as a shishou for that ability. That way future RPers who want to use that ability can receive proper training in it.
You are not required to post links to all the threads you received training in in your bios, however you should always be able to provide these links if requested by an RP mod when there is doubt you didn't receive training in a specific skill. So it's advised you don't make things hard on yourself by making several threads for the exact same training or train several skills in the same thread in an irregular order.
When someone made his/her 1st bio, instead of saying all the time "nice bio" or something along those lines, actually provide a more useful feedback in the case there is something wrong with the bio. It happens a lot of times that a person's 1st bio gets declined because it's simply not conform with the rules (non-Naruto history, incorrect template, etc.), but because 10 people said the bio was awesome, the creator thought he/she had done a good job and as a result doesn't understand why the bio got declined. You might think giving such a positive remark is helpful, but sometimes it isn't.
Never edit a bio once it has been checked and moved by a bio checker, especially when approved. When a bio is checked and approved by a mod, they're giving their seal of approval on the rendition that was approved at that date. Going back and editing anything after that point, even as small as a picture, would make the bio void and therefore is on grounds to be declined.
If you seek a permission for someone and are granted said permission, be sure to use it in a timely fashion. Despite not having a defined expiration date, you can't also claim half a year later that you gained a given permission. The ability in question might have changed or other factors may prevent you from using the permission after much time has passed. When you seek to earn or gain something its expected you actually pursue it. Also, permissions are only valid until you use them; once you've taken a battle test and fail, you'll need to seek the permissions again since you've already used them.