Great advice so far in this thread.
Back when I was working for GW I was doing the odd commission piece at decent rates. Nothing too complex, no NMM, basic bases etc and I was making around £200 a week in my spare time. After I had a few awards under my belt I was approached by an "agent" of sorts who had been in the mini painting biz for years and had a huge list of contacts. The first load of minis I did for him, a LOTR orc battleforce, about 20 orcs 10 uruk hai a lurtz and a cave troll went for $1600. Sounds good but asides from a few army orders there was never really a reliable constant demand. I stopped taking commissions for a few years and have only recently started painting again.
The main things I learned from this are -
1. Piggyback the painting on top of another job. You'll never make enough on it's own, even if you can pull down huge amounts for a mini, one change in the economy and you're sitting with nothing for months.
2. set a base rate. Stick to it. The amount of times I had someone approve a paintjob and then ask for major changes and then expect that to be covered is astonishing. That brings me to...
3. Nothing is impossible, it just costs more. Make sure everything is appropriately priced.
4. Keep a log. Note all of your paint mixes and processes. People always want something you've already sold.
5. Set a price list and stick to it. i.e. - Basic paintjob 1 level highlights and shading £20, 2 levels £25, freehand +£15 etc. People always want an incredible paintjob and expect to pay nothing. Let them know what they're getting.
6. Get a reputation, posting WIP's and painted mini shots here and on other forums is the best way to let people see how your work is rated by others.
7. Don't be afraid to walk away. Sometimes people will want to negotiate prices. Sure if you're wanting a custom converted and painted 2000 pt marine army at a high level you can throw in some freehand but when people are taking you below you're base rates tell 'em to go fish. Time wasted negotiating is time you could be painting.
8. there is a 50/50 chance that painting professionally will destroy any enthusiasm you have for painting and gaming within 6 months.