Just to add my two cents here but pretty much repeating what some of the other members said, I drop my frozen rat in the hottest tap water possible and refresh the water as needed. This whole process takes about 20 to 30 minutes. While this is going on, I'll give my BP a warm bath to get his appetite up and running. After confirming the rat is completely thawed, I dry it off with a towel and I use a blow dryer to bring up it's temperature some more. Moments before actually feeding, I focus the blow dryer directly on the rat's head to give the BP a focus point. This process works 95% of the time.
I tried using the zip lock method before but it seemed to slow the thawing process. Straight into hot water works for me. Hope my input helps.