There are tons of great tutorials on coolminiornot.com and Reaper minis' forums and main site, as well as a load of other blogs and minis-related sites around the net. Look for tutorials with "freehand" in the title. Most have photos so you can see which painters' methods give the sort of results you're looking for. Then it's just a matter of practice before it gets easy to pull off. Maybe Dark Sword's painting DVD has some freehand instructions too? Not sure, somehow I just heard about it for the first time today.