Good tips Spuda! Yes model paints like Testor's are known to have a "never dry" effect on the plastics Hasbro uses. They can remain tacky or even sticky, and are instantly full of finger prints. Acrylics are the way to go. Get some good versatile brushes. Use the finer tipped brushes with soft bristles for detailing, use large brush heads for general coating or large flat surfaces like on vehicles. I highly highly recommend using Citadel Paints. They're water based acrylics, very forgiving and come in the most unique colors. They do amazing metallic pigments too. They're what I prefer. One of their jars gets me through about 20-30 figures and some small vehicles or dioramas.
Another great thing to remember is addition and reduction work.
Addition= apply paint in coats/colors. i.e. your figure has a black jump suit, so you start by painting all the jump suit surfaces black. It doesn't matter if you get some black on other parts like his armor because those are going to be silver. After the black has dried you go in and add the silver to his armor pieces: chest, shoulders, knees, etc. Then it's time for
Subtraction= so you got some silver on the jump suit areas. No big deal. You take your finest (smallest head) detail brush, and subtract the silver out with some black. This is known as the "touch up" phase of your paint applications. Addition and subtraction work great for detailed figures or vehicles. It's a technique used by most model makers and table top gamers. From my Warhammer days we used to paint figures in batches. Start by priming them all black. Then painting the armor pieces red. Then the do the trim in blue. Etc. Etc. Until each piece is finished.
With Citadel paints you can even pop your figure appart, paint each piece individually to get a very meticulous and even paint job. Then boil the pieces (see Spuda's tutorial on the Boil-and-pop method) and pop them back together. This gives you a flawless coverage of paint. Citadel paints are water solluable (they wash off your hands, clothes, etc.) but once they're dry on a hard surface they are water proof, so you can boil them and not have your paint wash away.
Another handy trick is to sand certain pieces to get a porous surface to really grab the paint. This makes it that much harder for your paint to chip or flake.
One last tip comes from my days in the 501st Legion, and that's what they call the "50 foot rule". Essentially, how will your piece look from the standard viewing distance? As for the 501st 50 feet is what they consider event/parade distance. For action figures I'd say 2 feet is how people will normally see them on your shelves. For photos it'll be much closer. Ask yourself, "Will people see any unpainted surfaces? Will my weathering look as good from 2 feet as it will from 5 inches away?" and other things like that. I personally paint the backs of knees, under armor, the bottom rims of helmets, etc. so that all the little details are 100% relative to the custom.
Also take a look at your figures, Hasbro cuts a lot of corners with their painting. Sometimes it's the wrong color, it's only painted on one side, or they leave bare plastic where there should be paint. I have a Baron Fel figure that has exposed flesh toned plastic on the edges of his black gloves. They also don't always get the paint in the right spots.
Hope this helps