1. So when picking any pumpkin that I do, I try to make sure that I've already decided on my pattern. That way I have an idea of the general shape of how I want the finished product to look. So, for ex. the pumpkins I chose for Jack Sparrow and Davy Jones are long and somewhat narrow. The one I chose for Yoda was short and squat, and for Darth Vader I chose a larger, more intimidating pumpkin. Granted, the shape doesn't have to fit the pattern, but it helps with the overall effect.
In general though, MAKE SURE the pumpkin is large enough to fit your pattern without too much rounding of the edges. It can help to take an 8x11 sheet with you!
Beauty is (mostly) skin deep
2. Second thing I look for is the "face" side of the pumpkin. Which side do I want to carve on. It depends on how picky you are--if you don't mind a little "pumpkin acne" and realize that people will really only be looking at these pumpkins in the dark, it doesn't really matter if it's a little ugly on the outside. I try to find the most flat side of a pumpkin, or the ones that has the least ridges--makes it easier if you're doing a more intricate pattern. I admit that I like it if the pumpkin is fairly free of 'acne', but if it's a good shape, I can overlook a few flaws. Keep in mind, however, that the bumps can also be difficult to cut through, and murphy's law is that they'll fall at a particularly small piece of the pattern. The other thing you can do if you want a bumpy one is incorporate the bumps into the design.
Think outside the pumpkin
3. Now most people will stand at the pumpkin stall/boxes for a good twenty minutes and wage a small crusade trying to find the one pumpkin in the lot that will sit without wobbling, or have the 'good side' facing upwards.
Cut your pumpkin from the bottom instead of the top. It's a safer way to light your pumpkin if you use candles (far less burning of your hands), and easier to insert if you're using electronic lights. You can also then shape the bottom to get a perfect angle on whichever side you like. And you can afford to adopt some wobbly orphan pumpkins that no one else wants.