You can't upload much of what's rarely downloaded. The usual way to help the network is to download something popular that you don't need and to seed it. This removes some strain from big seeders and helps them seed other things you don't have.

But personally I believe you're not obliged to anything in torrents, especially with adsl. We got you covered with our multi-hundred u/d ratios. Everyone participating honestly and comfortably is enough, even if that means no upload. Just don't turn it off out of unnecessary principle after downloading.