When doing drybrushing, I recommend doing a \"wetbrush\" for the first couple layers, then a more traditional drybrush for the hilights.
Wetbrushing is like drybrushing, but you use 1-2 strokes with slightly watered down paint on your brush. Go ahead and wipe some of it off first. This technique takes some getting used to, but it can keep the texture from getting grainy.
For those hilights, be sure to wipe damn near every bit of paint off that brush first, I never wipe enough paint off and usually end up regretting it.