I hate when I remove my towel to wash, and forget to put a new one in there. In the winter, sometimes my nose runs for some weird flu reason, and I need to blow my nose. I need to keep a snot rag in my tennis bag.
I also hate when I forget a snack. I think I will buy a box of energy bars and just keep the whole thing in the tennis bag.
I hate when I go to tennis directly from work, and forget to pack socks. I will keep a few pairs of socks in the tennis bag.
I travel for work, so my tennis bag is basically another iteration of my laptop bag...basically a rolling wal-mart. what's funny is, when I'm home is when I actually am more prone to forget things. Usually if I do, it's either a towel or an extra t-shirt for after playing. One time I showed up to a match with two left shoes!
I have a Prince 3-racquet backpack style bag, which, at *all* times contains: :
at least two racquets + a demo to mess around with
clothing:
--several visors/hats...about 5 are generally in there
--wrist bands. they come in pairs, so i buy about 6 pairs and keep one of each in two separate ziplock bags
--extra shoe laces. never needed em for a shoe, but they come in handy to tie up untidy nets
--one t shirt is always in there, as well as a towel
accessories (I should buy stock in zip-lock bags): tourna grip roll, handful of dampeners, measuring tape, roll of athletic tape, a couple pens, gum, baggie with a stick of chalk in it, spare change, small pump bottle of insect repellant, lip balm, small flashlight, nail clippers, lighter, advil...lots of advil, cliff bars
side pockets: laminated 4x6 of The Code (given by my local assn), laminated quick ref card for USTA league rules/procedures, ace bandage, small first-aid kit
Also: thera-band for stretching, flexbar for elbow, elbow strap, thigh roller for cramps, 1-2 cans of balls, sunglasses, extra 'breathable' small bag for wet clothes
Most important besides my raquets? a couple of neoprene sleeves for cold beer -- 1 for me, 1 for my opponent.