The following is quoted from another member of bike forums, in reply to a similar post. I found it so insightful, I emailed it to myself, and now have had to pass it on several times.
"You can get a really nice paint job with rattlecans if you take your time. It won't be as good as a pro job, but good enough that no one will notice without a close inspection.
Contrary to popular belief, the difference between amateur and pro has very little to do with the paint itself. Yes, pros have better paints (most of which are too toxic for home use), but what makes the biggest difference is their prep work!
Painting only takes a few minutes, its the prep that takes time, and determines how your paint job will come out.
First off, unless the original finish is actually peeling or flaking off, don't sand or strip it off! Just clean it well, and sand it lightly with a 220-500 grit sandpaper, and dust it off with a terry cloth, that will roughen up all the surface.
Then, buy yourself a primer, a paint, and a clearcoat. I personally like Duplicolor. I use their basic primer, their basic enamels, and their clear Engine Enamel (its stronger than their regular clear). Read the backs of the cans. Seriously read them. No one does, but it really is important! If its too humid, the paint won't dry right. If you recoat outside the 'recoat window', your paint will bubble and crack. Read and follow the directions, it makes a difference.
Remember that three thin coats are better than one thick coat. Don't worry about getting complete coverage with your first coat. It won't happen, and thats OK. Just spray on a light coat, give it 20 minutes (or whatever is recommended by the can), and lay down another one. If it takes 7 coats to get complete coverage, so be it. I recommend hanging the bike up with a rope through its bottom bracket shell so you can freely rotate the bike around. Bikes don't have a lot of surface area, but they have a lot of weird curves and places for paint to miss.
If you want your paint job to be extra nice, once you are down laying down color, give the frame at least a full 24 hours, preferably a little more, and then wet sand the frame. Use a really high grit paper, like 1600 or up, and put a drop of dish detergent and some water on it, that lubricates things and keeps things more even and smooth. Don't try to remove paint, just smooth it out. Then rinse it clean, and dry it carefully. Then you can move to your clearcoat. If you don't want to wet sand, thats fine, it'll still be good.
I usually lay down at least 3 coats of clear. Make sure your last coat is a 'solid' coat, not too thick, of course, but don't just dust on the last coat, or you can get little flecks on the finish. Let it dry for at least 24 hours.
To finish, I use some automotive polishing compound on a terry cloth pad, and work it around by hand. Rub rub rub, but remember, you aren't trying to remove paint, just smooth it out. Wipe it clean with a clean cloth, and then apply an automotive wax, like Meguirs, or even cheap stuff like Turtle Wax. Buff it well, and enjoy.
If you can stand it, wait a few more days before building up the bike, enamels take a long time to cure, even if the paint seems dry, it will be most likely to chip the first few days.
Anyway, like I said, it takes time, but is worth it. I've painted quite a few frames and other things with this basic method, and it never fails to please."