What Mike said. To elaborate, wrapping in double heavy foil, then an old towel, then in the (at least room temp) cooler - what has been referred to as FTC (foil, towel, cooler) can keep it getting tender as it rests and hot enough to serve for a couple of hours at least. BTW, just for planning whether you'll be able to go to bed, at 225 degrees I'd plan on at least 24 hours. The "stall" where it sits at 165 or so for a long time, will take several hours so don't get discouraged or tempted to turn the heat up. PS - make sure you put your probe on the fattest part of the flat, not the point. Sorry if I'm telling you stuff you already know. If your question was specific to what to do with prime brisket, as opposed to choice or CAB, I'd probably look for slighty less overall time.