Hey mate. I used Krylon camo paints (available from Walmart, etc). I didn't prep the weapon (as in rub it down, etc), just taped over the bolt as seen from the ejection port and, plugged the barrel and mag well with newspaper. In addition, I masked the iron sights just to make them stand out (up to you on that one though). I never had a huge gap between my upper and lower so never bothered filling any gaps (but you might want to).
I'm not a fan of mimicking a pattern (eg, Multicam), as I don't like the looks of those paint jobs when they wear down (which it will). I prefer strokes / snake skin, etc, that will not look so out of place when the paint inevitably does wear down / off.
I won't post a pic of the weapon, as there is a thread for that, but here's my flickr.
smith & wesson m&p15-22 with primary arms micro dot | Flickr - Photo Sharing!
Good luck, have fun and post pics when you're done! Make it yours!