If you use a brush that has a lighter opacity I usually try to have a brush that is sort of CHALKY then I'll space it a bit so that it's not so filled and has a bit more randomness to it. I tend to use a yellow color when initially chalking the edge so that i can change the color easily later using the colorize option or replace color tool in photoshop.
It takes a while for it to look right but then i keep the template and i can recolor it to anything later if i want to use it again.
lot's of great brushes can be found that are already "grunged" up or you can make your own.
Photoshop has some default "chalk' brushes that work ok as well. Rotating your brush too as you do each side so that you are using the same 'edge' of the brush along each side.
I hope this is making sense. --

hope that helps a bit