For straight edge camo, just cut out different sized masks for masking tape. Make sure to burnish the ends down really well, other wise, pait will seep through. For soft edge camo I use an airbrush. If you are working with a spray can, (heres an idea, I havent tried it, but it might work) then you could use rubber cement.
spray the camo color I.E. dark green. Mask off the areas with the cement. Spray the base caot color, and then pull off the mask. Once again this is only a hypothosis. I have only used rubber cement for paint chipping effects.