Cels Forum

Well, for about $5 you can take a cel painting workshop
at most anime conventions; it's definitely worthwhile 
since that not only covers the supplies (acrylic paints are
very expensive) but gets you in a room with a professional,
someone who should be able to answer all your questions and
save you a lot of trial and error learning...
The stuff I remember from the workshop I did (it was fun)
is- 
Paint from left to right, top to bottom.
Do small detailed areas first.  
Make sure the paint coat is thick enough that when you hold the cel up to the light, nothing shows through. (For me a second coat worked best here, as well as slightly thicker paint).
And I think it was do dark colors before light because they
dry faster, but I can't remember that one for sure.