@goldenmist - Synthetic brushes are fine to use also. The most important thing is that the brushes are flat and if you want to do precise details you're going to need a small brush, a brand new #1 or #2 would be ideal. You might need to replace these every so often because they tend to lose their edge. Of course you can also use bigger brushes to cover larger areas.
Another tip is to use a kneaded eraser for highlights. Don't forget that dry brushing isn't limited to portraiture either; it's great for blending lines and background details as well. It’s a very handy technique to learn if you want to be a painter.