A possible suggestion

I noticed that basically all the torrents I've tried downloading, except the newest and smallest, always get stuck at some % with no seeds.

So maybe to encourage seeding, you could make it so that seeding a file you have already completed counts extra compared to uploading during the download or something? Or perhaps put a subsidy on torrents that have been stuck for weeks so that a person who seeds them gets credit somehow?

If you re-seed something you already have, then your ratio will exceed 1.0, so there already is an insentive.

In short, re-seeding assures you will have good ratio, and it lets you abort things you find out you did not like.

The real issue is that many people transfer files, read the boards, etc for a while, then lose interest. They stop using the forums, and just use the tracker.

There is no practical way to get people who are no longer reading the forums to do re-seeding. They would not even see any information on a new insentive.