I had this problem but not on a seat post, but a piece of equipment I was trying to qualify. You can get a set of feeler gages if you are feeling (haha I made a funny) like you need to find the exact gap, or you can go with Teflon tape. Find where you want the seat to be and mark that spot (that will be inside of the tube spot) then wrap up and down about 1/2 inch on either side of the spot with Teflon tape. It sounds like you have a gap and Teflon tape, while slick, will often fill the gap as it is thin. I find electrical tape might be too thick. It also might be that the tube is opening a little while you ride due to the stresses on the bike, which your bike shop should ask Trek about. Did you look for any (I'm sure you have) damage to the tube like a vertical crack? These sometimes are invisible and when you apply force to the seat and stir it around, it will open up (this happened to my first Gary Fisher I had, I guess it got mushed in transit). In that case I returned the bike and got a new one, but you can't do that here. Hope you figure it out. That sucks. My new GF does it once in a while but it is because I haven't cranked the clamp down.