I like to airbrush Tamiya acrylics thinned with rubbing alcohol onto a primed surface. I prime with Plastikote T-235 grey sandable primer - it sticks to everything (resin, styrene, vinyl) ensuring a uniform base for your acrylics. Be aware that it is a laquer based rattlecan. I have a spray booth in my studio that vents to the outside, but if you spray without a good vent, the fumes build up pretty quickly. I swear by this primer.
If I'm brush painting, especially figures, I like to do the heavy lifting with Vallejo paints (on primed parts, of course) as they seem to be heavily pigmented with less binder than other brands, have superior opacity/blend-ability, and "flow" well. They are acrylic based, and can be extended with retarders. I usually go over that base with a few oil-based paint passes, building up skintones or other surfaces in a glaze. I prefer to use Winsor & Newton Artist Oils for this, and will thin with either linseed oil, damar varnish, or if I'm lazy, xylene - the xylene will almost make the oil behave like an alklyd, drying in much less time, but losing some of it's glossy "depth".
I also "brush" with powdered pigments, but I guess that's more along the lines of weathering and not painting.
If you really want to see a completely alternate approach, you should see what the Japanese artist/Scale Aviation editor Kow Yokoyama does with lacquer-based paints - all brush painted 1/32nd P-40:
http://homepage3.nifty.com/kow/P40B01.html
I asked him about his methods when I was in Japan a few months ago, and he told me that he approaches a model as if it's a canvas, so you end up with a three dimensional painting. It's a very "painterly" and (to my eyes) unique approach. And man, I gotta tell you, it's hard to attempt. I tried to unlearn my personal way of painting, and try the hand-paint-everything approach on one of his non-aircraft kits. Boy, did I make a mess!
