Hey SG. I don't know your level of skill, so I'm just gonna tell you all the basics. I'm not trying to be condescending. I hope this gives no offence.
If you're painting w/ brushes, I offer the following advice:
Invest in a good set of brushes. This investment pays dividends on every model you build.
Red sable brushes are best, but nylon brushes are cheaper, and an acceptable alternative in most cases. Take care of your brushes, and they will be good to you.
Never paint right out of the bottle. The paint is never the right consistency, and having an open bottle causes it to begin to cure, which will eventually leave you w/ a bottle shaped lump of dried paint.
Instead, get yourself a paint palette from a craft store, the kind w/ little separate cups. They cost like $2. (There's a picture of one in Buddho's "Unknown Starship #2" thread, in the background of the fourth and fifth picture down, on the first page.)
Put a few drops of paint on the palette, and close up the original container. Thin the paint w/ the appropriate thinner, so that it flows off the brush, but not so much that it starts to get watery. there's no formula for this, you just have to eyeball it. Keep a paper towel right there, to soak any excess paint off the bristles. Paint w/ fluid strokes, in one direction only.
Paint only one coat at a time. Going over wet paint w/ a brush will only create ‘brush-strokes' that might have to be sanded out later.
Allow the paint to dry completely before adding another coat. If you're using acrylics, this could be minutes. For enamels, it could be 24 hours.
If the paint is still transparent after it dries, apply another coat, in the same way, following the same direction as before. Never ‘cross paint'.
The nice thing about painting w/ a palette is that you can mix small amounts of paint right on the spot, for shading and highlighting effects.
Let's say, you're painting a set of grills/louvers. You paint them a medium grey (for instance). You can then take the same grey, mix in a little black (30% or so) and then paint the inside of the grill louvers, wherever there would be shadows on the ‘real' thing. Dry brush w/ the original color, and now you have added depth to your paint.
Then, mix some white w/ the original grey (again 30% or so) and drybrush the highlights. This gives a much more three dimensional appearance to your work, and its super easy.
Finally, if there is any way you can swing it, get yourself an airbrush. It will cut your painting time in half, and get you better results.
Hope I helped -JC