I can't usually whistle very well after practicing either, but I think it's just because the apperture gets a little puffy and the whistle doesn't happen as easily. Not puffy enough to effect the trumpet, but the whistleing mechinism seems a little more...fragile? I don't know. One of my teachers liked to use what he called a "ghost whistle." I don't know how to describe it exactly, but you blow like you're going to whistle, but you don't quite let the whistle happen. It really quiet and sort of just sounds like the whoosh of air, but with a pitch. He used it to hear difficult entrances (it's so quiet, the guy next to you wouldn't hear it) and he also thought the tongue action was similar enough to the trumpet the you were setting up your ear and body to play the right note. It's kind of a cool little practice aid. J.