Be patient, even if it means falling in line for a while. Wait for the group to slow after a hard pace and then instead of sitting up like everyone else, put in a few more strokes. You'd be surprised how many places you can advance by doing this.
I've found the best times to advance are right after primes when everyone sits up after going flat out for a lap or more if you can't do it comfortably and safely through the turns.
Don't bomb the corners i.e. take the extreme inside and chop wheels to advance your position.