by past nastification
I've yet to cross the line into the high-end acrylics aimed at hobbyists/model builders.
Instead, I stick with the $.95 bottles of acrylic paints brands (like FolkArt, Americana, Apple Barrel) found in stores like Michael's or other more "craftsy" stores. I dislike spending money at Wal-mart, but they also carry these paints. I've racked up so many tubes of these kinds that it would feel wasteful for me to switch to an entirely different variety. Plus, I'm used to working with them.
The best flesh tone I've found is FolkArt's "peach perfection". Adding white for pale flesh tones works well, as does adding brown for darker flesh tones.
If you're bound and determined to mix your own flesh tone from scratch using primary colors, try a ratio like this:
white:yellow:green:red 10:3:2:2
Or, if you're working with colors you've already got but can't make a flesh tone that looks right, try adding a very, very small amount of either green or blue.
Good luck!