Oliver does this too!!! Drives me crazy! He's 23 weeks old today and we've had him since he was 9 weeks old so I don't think it has anything to do with being a shelter dog. Anyway, hopefully you get some great advice on this as I could use it too. "Drop it" works on occasion but my dog thinks of it as a game. I accidentally taught my dog a game where he holds a ball or kong in his mouth and I try to smack it out and then he'll go chase it as it bounces along the ground. I am pretty sure that is where he has learned to back his butt up to me when he has something and not let me get it from him. He'll come in from outside and vice versa with no problem, thank goodness.