You defrost when using Flame wheel or Sacred Fire, or from being hit by a fire-type move, and of course, a status problem preventing move, like Aromatherapy. Also, I believe Sunny Day speeds up defrosting. Magma armor prevents freezing. Other than that, YER FROZEN! freezing is very nasty, watch out. I heard you can stay an icicle for the whole battle! And there's never a limit to freezing. either that, or you can stay frozen for up to SEVEN turns! Pack your Aspear berrys!