I really really really really (x infinity) hate doing flats... but it's one of those things that fall into the 'you want it done right, you do it yourself' category. I find since everyone has a different way of doing flats, you never get a flatted piece the same way twice, so it's hard to put down a constant rhythm.
Also, I 'paint' my flats by hand. -always-. I do not use a paint bucket or a fill tool, I do them all by hand, painting over the part I'm flatting with a large brush, and then going in with the eraser and erasing all the bits I don't want... with occasional touches-up with the paintbrush again if I accidentally erase something I didn't want to.
I put each colour on its own layer, except for the girl - since she will have so many different colours, I paint one solid colour for her (in this case, skin, since that is the most dominant colour on her), and then I'll place the different colours for her outfit, hair, knife, etc. on sepperate layers above her, with a 'clipping mask' enabled so that the new colours don't stray from the one flat colour I already have on.
I know that prolly sounds complicated, but that's what it is. lol.
Anyway, I've decided that in order to fill out my gallery a bit more, and to enable me to make semi-regular updates instead of just updating when I post a finished piece, I'm going to post works in progress.