What I actually used to do is paint most of the pieces while they are still in the mold (after removing any unwanted lines just like Flyboy said) and then after assembling them I just touched up that little dot left over from the mold. I removed the pieces with a sharp utility knife.
Do be exact, I took all but two or so connections to the mold off (preferrably two on the bottom of the fuselage) on the big pieces.
That way I could hold the pieces by the mold and it gave me more of a chance to avoid streaks and fingerprints. Also, with matt finish, it's easier and luckily most army helicopters and military planes I built I didn't have to go gloss...
I don't usually paint over the decals when the paint underneath isn't gloss either.
For the playing cards on the guys helmet I used a toothpick to make the dots on the card since there are no brushes that small.
One trick I learned over the interent for making dash boards I used to give the model the chipping paint look. Put your brush in silver paint, wipe the paint off just leaving a small bit over it. And wipe the brush over areas where the paint on an airplane would wear out first. Just lightly. This leaves you with an intersting weathered look.
That's a very good trick which I also use on steps, or the helicopters skids, or around the filler post to show scratches.
I also used to dilute black paint until there is almost nothing left and applied this thin runny mix to get the appearance of oil stains.
For exhaust streaks (i.e. on the belly of a Cessna I once made, or at the wingroot of the Corsairs which you always see on the real planes) I used airbrush with black paint (or green/brown for the corsair)
The list is long! I had a lot of fun doing this back in the days but looking back at it, it might have been the glue I used because it does seem very tideous !!
