The thing that helps me the most is a magnifying lamp on a long arm. It allows me to see when the brush is getting close to the surface as well as puts a lot of light right where I need it. I actually have two of them, one is fluorescent and cost $50. The other is incandescent and cost $20. I seldom use the fluorescent one and much prefer the cheap $20 lamp.
Next, nobody's hands are really still enough to paint detail correctly. Hold the brush like you would a pencil, down near the tip. Brace your hands on anything you can find to brace them on, and turn the parts so your wrists aren't strained (strained muscles mean shakes). Paint with somewhat thin paint, use the brush to get a little paint on the part you are painting, then use the brush to push it toward the edges. If you go over the edge a little, don't worry about it. When it's dry use the base color to touch it up again. The closer you can get the edges to correct the less you have to depend on a wash to cover up mistakes.
Some acrylics do not like to be painted over. Specifically Tamiya flat black. Paint over it and the new paint will dissolve what's there. Know your paints and what you can and can't do. If necessary put a layer of Future between colors or use enamel if necessary. I have recently gotten into the habit of painting areas that are going to have a lot of detail with a gloss enamel base (or flat enamel with a coat of Future) and use acrylics for the detailing. If I mess up with the detail it usualy isn't real hard to get the acrylic off without messing up the enamel.