This is a common "complaint." A lengthy discussion can be found at http://kfs.infopop.cc/eve/forums/a/tpc/f/5086057385/m/7461070694?r=2481080994#2481080994
The most severe solution is to pound (use a mallet) the seat pegs into the peg holes. This is sort of a permanent fix, which I don't like because I like to remove the seat after each trip. It also seems to me that pounding the pegs into the peg holes may sooner or later cause those holes to fail (which they are known to do on occasion) with subsequent major leakage into the hull.
One person on the above link suggested (actually his wife suggested) putting a piece of kitchen-drawer, non-slip pad under the seat. It worked for him. I tried it. First, I removed the pegs, then put the rubberized non-slip pad down. Here is a picture:
The seat does not slip with the non-slip pad.
I have suggested elsewhere that Hobie should get rid of the seat pegs and the problematic peg holes in the hull. Then, put a non-slip bottom on the seat, and, EUREKA!
, the proplem of the slippery seat is solved.