DON'T WAIT TOO LONG! When you start feeling like you might have to go soon, don't procrastinate! If you wait until you you're about to burst, and then have to stand in a long line for the portos... It's gonna be the longest, most miserable 15 minutes of your life.
KNOW WHERE THE PORT-O-POTTIES ARE. Study the map and see where the groups of portos are. In 2006, there were 3 main locations inside the festival: 1) Just inside the entrance, 2) next to the Outdoor Theatre stage, and 3) Between the Sahara tent and Mojave tent. There's also several outside the festival, near the entrance. In the afternoon as people are arriving, the lines for the portos just outside the entrance and just inside the entrance can get really long. It seems the first thing most people want to do when they get inside the festival is pee. Keep walking past the long lines at that first group of portos and make a beeline for one of the other 2 areas, where lines are shorter.
SLIP AROUND THE BACK. The typical configuration for the 3 groups of portos inside the festival are 2 long rows facing each other, covered by a sun shade and forming a sort of "hallway" so to speak. When it's busy, people tend to queue up at the front of this hallway, but sometimes you can walk around the whole thing and sneak through the back opening. Worth a try, especially if you ignored Step 1.
BABY WIPES ARE YOUR FRIEND. If you've read my "How to Stay Clean at Coachella" article, you already know how I feel about handi-wipes or baby wipes... they're a must-have. And they can be very handy when you're inside of a porto. You can use them to wipe down a seat before you sit down, clean your hands when you're done, or even use them to... umm... wipe yourself if you find yourself in a porto with no TP.
YOU CAN'T PEE IF YOU CAN'T SEE. After the sun goes down, it can get pretty dark inside some of the portos. The upside to this is that you won't be able to see that horrific visual when you catch a glimpse of what's inside the tank under the seat (it's enough to give you post-traumatic stress syndome, I tell ya!) The downside is you can't see what the hell you're doing, and could end up peeing all over your shoes, or touching something you really don't want to touch, or worse. If you have a glowstick or a little penlight or LED that you can wear around your neck or clip to your shirt, it can provide just enough illumination to let you see where you should aim. Or sit. Or hover.
THERE ARE A FEW "REAL" TOILETS. If you've led a very sheltered life and the thought of using a port-o-potty is just too awful, there are alternatives. There are usually a couple trailer-style portos (each with a men's room and women's room) with flushable toilets. For the past couple years they've had one just inside the main entrance, and I think there might be another around somewhere too... not sure. Naturally, the lines are longer for these, but for some people it's worth the wait.
IT'S MUCH BETTER IN VIP. You get what you pay for, and if you can shell out the benjamins to hang with the beautiful people backstage in VIP, there's a definite step up in bathroom quality. The port-o-potties there are in trailers, and are like, real bathrooms, with stalls and sinks and soap and stuff. Oooh-la-la...