If you're using Vuze, then once you've begun downloading, click on the downloading tab on the left menu, then right click on the download and select "show details".
Once you're in the advanced details, click the tab in the top left of the download. This is the download/upload speed limiter (0 is unlimited). In "max download speed" enter "1". This will limit your max download speed to 1 byte (or kilobyte) per second, and leave the upload speed unlimited. Not perfect, but close to it! Hope I could help
