I use Silly Putty for hard edge camo lines. I find it lets me be more creative with the camo effects than doing it free hand. I can basically see what the pattern will be before I apply the paint and make adjustments.
I'm not sure how it would work for soft edge. I suppose it would let you control how far out the overspray goes, but then you would have a point at which there is no overspray and the transition would be obvious. I think you want to do soft edge without the use of masking.
For hard edge, though, Silly Putty is great. Do not use rattle cans with it, however. There is some chemical in the rattle cans that interacts with the Silly Putty and turns it to mush. I seem to recall I have had a similar, but not quite as extreme, experience when I used enamel paint with Silly Putty. But for acrylics, Silly Putty is pretty great stuff.
Now, you will run into a different sort of problem when you use this technique--the lines are so clean and crisp that they look too good. You'll want to either lighten up your paint so that the color differences aren't so stark, spray the paint on in a thinner layer so you don't get the full effect of the darker color, or go over the entire model with a light coat of something to get all the colors to blend in. For a good example of before and after, you could go to my King Tiger Work in Progress in the Armor forum. I have one set of pics with the hard edge camo and then another set after I sprayed over the body of the tank with a lighter color to create a faded effect. It also has the effect of blending the colors some so they look more realistic together. You can control how much "fading" you want to do simply by applying more or less of your top color. You can also "fade" surfaces that would have been exposed more to the sun with this technique. Just spray more over some spots than over others.