I'm no expert at painting, but I have rattle-canned a few frames. Here's what I have learned:
1) Surface preparation is critical. This means, clean, no grease or oil, or even fingerprints. A phosphate etch or light sandblasting is helpful. Handle the clean, prepped frame with clean cotton gloves before paint to avoid leaving skin oils from fingerprints.
2) Fresh paint is VERY fragile, even when apparently dry to touch. Do not clamp anything on the frame where it will leave a mark -- this includes repair stand clamps. Put a seat post in the frame and clamp on the post instead. Baking fresh paint will help it harden faster, but few of us have an oven big enough to accept a bike frame. If you have an attic, leaving the frame in the attic for a few sunny days is also often effective.