Drybrushing: This is a pretty quick and easy technique, that while not as clean as others, produce a nice effect for highlighting. First you need to identify the color you are going to use as the highlight color. You need to go to a lighter color then your original if you are going to highlights, though you will find times where you need to use a darker color as well, when place shadows is a good example. You can get the lighter color by adding white or bleached bone to the original color if your paint pallet is not that expanded.
Once you have the color you are going to use you need to grab that brush, do not use a brush that you are going to use for other purposes, once you have picked a drybrush its pretty much going to be ruined for any other thing. (This is also why you never throw away a paintbrush, they always have a use).
First you need to put a glob of paint on a pallet, this is much easier. Drag a bit of paint out and make a slight stabbing motion with the brush, twisting it while you do it. This causes only this tips of the brush to be covered.
Then you will want to wipe almost all the paint from the brush. I normally do it by quickly and lightly flicking the brush over a piece of paper towel. You will then want to add a bit more pressure as you only want the brush leaving very little paint. The more you wipe off, less paint you will put on the model. You can use this if you are layer paint to a faded effect.
Now its time to go after the model. I always go from the top of the model down. This way more paint is on the top of the model vs the bottom, giving it more of a natural appearance. You want to gently rub the tips of the brush over the model, only leaving paint on the raised part of the model. You can always go over an area more then once to give it a nice paint build up. Just keep in mind that light comes in from the top, so those areas will be lighter.
Note: I used green so it would have a high contrast and show up as my normal drybrushing is done in about 5 steps.