Be patient
Applying an even coat of paint on any surface without making any mistake is going to take a while. You have to make sure your strokes go in one direction. And make sure to stay within the lines, so to speak.
Don't be too generous with the paint
If you put on too much paint, the extra paint just clogs up the tiny crevices. When they dry up, they blot up all the details. Plus, you waste paint. You're gonna have a bad time.
As soon as you touch that paint to plastic, you're committed.
When you've started, better to do it well and you can't just quit mid-way and change colors. I mean, sure, you can, by "erasing" the bits but it leaves some color and it gets weird. Unless you've got some paint thinner, erasing partially dried paint bits will leave some paint.
Plan ahead
This is my mistake with the HK416. I was all "Meh, wonder what it'll look like all in gun black" and now even the goddamned pistol grip is gun black. I ordered a new one and this time, I'm just going to paint the metallic bits between the buttstock and the rail system.
Look at how much cooler the M60 Mk43 and the shotgun are.
Not all paint is the same
Are you wondering why my beautiful M16 is in that shitty-ass camo style finish? It looks like someone dipped it in a mudhole and let it dry. Well, I made two mistakes here - I used "flat earth" instead of "flat khaki" like I originally wanted. And another is that I didn't realize that the brown paint (probably all lighter shades) is really runny. So I couldn't get a coat thick enough to cover the black without it running all over the place. So find out the consistency of your paint first.
Type of plastic
This is unproven, but apparently, when you paint soft plastic, the paint chips off easier. This is opposed to painting on harder plastics, of course. This is how I formed this theory.
The accessories that come with G.I. Joe figures are sometimes comparatively softer than Marauders guns. I painted Stalker's bolo and Flint's Spas-12. After curing it for 5 hours or so, I put Stalker's bolo back into the scabbard and all the chrome paint just peeled off. I made Flint hold the gun and while I was doing that, a lot of the paint was rubbing off on my fingers. This has never happened with the harder plastic-made Marauders guns.
So, there you have our first experiment. I'm going to try and do a ton more as I get more colors and I play around with more guns and accessories. I'm thinking of redoing the M16 and the HK416. Hope to get better by then.