I like to cook some popcorn, make sure my DVR is set, and tell the kids that I don't want to be disturbed, especially if Murray is losing. (You did post this in the professional section after all) :-P
OK honestly?
Lots of water. I like to warm up a lot, maybe meet a friend at the tournament to warm up with prior to my match.
Have you seen Something About Mary?
love to watch pro strokes on youtube...visualize myself playing just like my favorite player...make sure to stretch and get moving my feet before I hit the court...watch Nadal as they are tossing the coin before a match...you can tell he has been getting warm for quite a while before he plays the first point