The excellent Pokemon TCG blog SixPrizes.com (a very good place to go for deck-building advice) has this article detailing essential items to pack for a tournament. I pretty much agree with all of them (except for the Battle Dictionary, which is only really needed for an international event such as Worlds or if you're attending a tournament in a foreign country), but I can also think of a number of other good things to take with you:
Hand sanitizer. When you're in a room with 30+ people who often handle each other's goods, you're bound to catch some weird bugs. Better to be safe than sorry.
Gum or mints - for you and for others. Proper hygiene is greatly appreciated.
Spare decklists, pre-printed or otherwise. Saves you registration time.
A good luck charm, if you wish. Nothing too adult, however.
Healthy snacks like trail mix or carrot sticks and a refillable bottle of water. Big tournaments can often go on into the night, especially if you're in top cuts or the side event.
Money. Quite possibly the most important thing of all. Most tournament venues have vendors on site, and a lunch break is often provided mid-tournament.
If you have any more suggestions, feel free to leave a comment!