If you want to gain ratio quickly, simply sort the torrent list by leechers. Voila! Your busiest data "markets".
Honestly, though, the only truly reliable method is to seed a relatively large number of torrents, 24/7/365.25. For example, I'm currently seeding 194 BakaBT torrents, of which 15-20 tend to be active at any given time. This easily uses up as much of my upload pipe as I care to give to uTorrent, and I have a decent ratio. Note, however, that I also do not choose torrents for popularity/"seedability"/Freeleech status, so YMMV; if you care to bother with it, you'll probably do far better than I do.