Wow, great tips everyone! I have a few comments/ideas..
I hate stitch and row counters. It's too easy for someone (old or young) to come up and say 'what's this?' and then fiddle with the dial.
I keep pencil and notepad next to me while knitting patterns I need to keep track on. If the pattern has a 4 row repeat I make hashmarks for each row then move down a line for the next repeat. I also circle the row that has the lifeline on it so if I have to frog I know exactly where I was in the pattern.
I second the crochet hook, too. Very handy for those dropped stitches!
adding to my previous list -
3. Keep coilless safety pins in your bag. They are great if you have to mark something below the row you're one or whatever. You can get them in the beading section of stores.