I attended La Tomatina 2 years ago. I'd suggest you take a few things under consideration:
1) There are a TON of people. Campers, drivers (I slept in a car), and busload after busload of drunken Australians (sorry, but there were!). As such, there will be traffic delays, and buses will be jammed.
2) While La Tomatina was fun, in Requena nearby the night before is the Wine and Water festival. You take a glass/vessel and parade around town, yelling 'agua, agua!' and residents open their windows and poor water on the crowd. After a while, a truck pulls up and you clamber onboard and use your glass to scoop red wine out of barrels on the back! And then this repeats for about 3 hours, fantastic fun!
3) Be VERY careful about your posessions. Pick-pocketing is rife - I had velcroed closed pockets and still didn't feel a thing - phone, money and rental car keys, and judging by the line at the police station, it was a big pay day for the thieves!
4) Finally, have fun - and as a top tip - the goggles, they do nothing! And also, at the end walk down the hill - there's a river you can wash yourself off with ;)