if you're going with 50%, can you set the first click to always go to a gallery? surfers who have seen something they like are much more likely to keep clicking. also if your mgp is a thumb preview type, you should mostly trade with other thumb preview mgps, but if you are using text descriptions, you should try to mostly trade with text description mgps.

something else that greatly affects productivity with thumb preview sites is the quality of the thumbs. they should be interesting, not blurry, not oversharpened, and something that you really DON'T want is to have a few yellowish or reddish thumbs in with the others.

btw, three trades probably isn't enough to keep things going.