1. Don't assume the painter knows anything. Try to provide a picture backed up by specific text. The painter may indeed be quite knowledgeable, but they're thinking the 1809 Uniform A when you really mean the 1812 Uniform B. This is especially true if your unit includes figures that vary slightly, such as elites in different headgear.
2. Be precise. Don’t say “yellow” if you mean “that chamois/yellow-green you see in this picture.”
3. If possible, sort and package your figures yourself, with the painting instructions attached to each group of figures. I have two regiments that were somehow reversed in the painting process (Regiment X figures were painted as Regiment Y, and vice versa). Not a huge problem and it's getting fixed, but if I had actually attached the painting instructions to the figures, it might have been avoided.
4. Pack your figures yourself. Be especially careful with bayonets, swords, and plumes. The painters probably can’t fix it if the figures arrive broken.
5. If possible, send a small unit or buy a sample. Pictures on a website help, but there’s nothing like having a figure in your hand or standing on your table for inspection.
That said, I have two painting services that I would like to recommend. There are others out there, and when I start posting all of the units I will show which service painted which unit.
First is Dragon Painting Service in Hong Kong. They sell pre-painted units on Ebay in a number of scales and eras. They are relatively expensive, in the the $7 US to $8 for a foot figure, including the figure. See the cuirassiers below. Turn around for special orders has averaged about two months.The other recommendation is Fernando Enterprises of Sri Lanka. I think the figures on their website don't look nearly as good as the figures they have sent me. Price is in the US $1.50 to 3.50 range, NOT including the figure.