More of a quick tip than a tutorial, but since I hadn't been able to find any good resources for this (everybody seems to say 'just use Photoshop' when asked), and I finally figured out something that actually works pretty well I figured I'd do a quick haphazard video tutorial:

Feel free to check it out

I'm using GIMP 2.6 because the GTK library used in 2.8 has some issues with pressure sensitivity not working, but the fundamentals will work in whatever version of GIMP you're using.