i'm actually going to be doing some freehand on one of my bloodletter standard bearers tonight, i'd be more than happy to post pics here of the process.
some general tips though-
1) if doing something complicated (like a dragon face), i like to lightly sketch the picture onto the model with a pencil. this will help get the positioning right. to change it, i just use a moist paper towel to wipe it off.
2) use extremely thin paints (a little more so then regular). this keeps the transitions smooth and is easy to correct. it will also soften the outline of the image and avoid that "amateur free-hand look," while keeping the colors nice and smooth.
3) pay attention to the order that you work in, as its NOT easy to go back and change stuff. work from the background foreward and dark to light. its obviously very easy to paint a gout of flame around a dragon's head if you do the flames first, the other way around is near impossible.
4) brush size that i found work's best is 3/0 or 000
5) don't be afraid to deviate from your original plan. i'm not kidding, 90% of your best ideas will come to you while working on it.
here's some of my freehand, starting from oldest to most recent:
