Take lots of water. Beer is a bad idea, not least because it can be hard to find anywhere to piss. Take sun block, too. Remember that you won't be able to sit down and recuperate whenever you feel like it, and think about how you'll pace yourself over the whole weekend. Sleep will become a precious commodity.
Glucose tablets are a good idea, because there are also few chances to eat.
Veterans take a sandwich or some fruit. The BVG issues a special Love Parade armband. In 1999 this cost DM10 and allowed the wearer unlimited travel on public transport for the weekend. This really is the best way to get about. You won't be able to park a car anywhere near the Parade, and traffic is disrupted from Saturday morning until Monday by scattered patches of dancing in the street.