I'll let someone else answer the materials 
question since I tend to use 'whatever works'  
but I will give you this tip: 

let your background dry for a few days before you 
put it in a cel book or anything like that that 
would press your cel against it for a while.  The 
stuff on there might be dry to touch, dry to 
scan, etc, but unless you let it sit for a few 
days it's a *lot* more likely to get stuck to the 
back of your cel.

Other than that, if you discover you're as crappy 
at watercolors (the usual medium of choice) as I 
am, I can give you some pointers on how to make 
something look like watercolor with the messy 
scribble of a couple of art markers and some 
liquid acrylic. ^_^