Thet guys have covered most of the pitfalls already, but I'll throw in a few more tips.
1. Use LOTS of wax and grease remover all through the prep and final prep stages. Put the stuff in a spray bottle, spray a small area, and wipe it off immediately with CLEAN paper towels. Don't scrimp on the towels, so if it takes a full roll, then use that many. The idea is to float any wax or grease to the top of the liquid, then wipe it off before it settles back on to your bodywork. Don't touch anything with your oily fingers after the final W & G remover cleanup, and allow at least 1/2 an hour for it to dry properly before spraying sealer or paint.
2. Go out and buy a painter's suit, with the hood, for your painting. You know, the white paper type with the elastics around the wrist and ankles. Most all of the contaminants that show up in a paint job come off the painter, not the car - wasps and bees not included. They ALWAYS land on the hood in your last coat of paint or clear.
3. Get a good 3M type respirator, with the proper charcoal filters and pre-filters, and wear it from the time you start mixing your paint until you are outside the booth. Also, go to Walmart and get a few pairs of cheap swimmer's goggles, and wear them while you're painting because isocyaniates WILL be absorbed into the eyes from the paint spray.
4. Use the slowest reducers you can when you're spraying clearcoat. I always use one step slower reducer that the actual temperature calls for so I get maximum flow-out in the clear.
5. Read and understand your material P-Sheets before you use any paint materials. I always have a printed copy of the sheets on my paint mixing bench, even though I've been using some of these products for the past 40 years. Cheap insurance.
6. DO NOT rush your paint job. If the P-sheet calls for 20 minutes between coats, wait at least that long. I usually give it an extra 10 minutes to be on the safe side. If you rush the job, and most first timers have a tendency to do so, then you are going to trap solvents in the finish, and believe me, they will come back to haunt you. And always try to spray in the morning before the bugs wake up to ruin you fresh paint.
7. Your final paint job will only be as good as the bodywork underneath. If you think you see a small imperfection, and then decide the paint will probably hide it - WRONG! The final finish will only amplify any imperfections or flaws in the bodywork, so make sure it's perfect before you even buy your paint.
8. You can paint it yourself, so get 'er done, and have fun.
