Don't overload the brush with paint, dab just the end of the bristles (hairs) into the color you require. Having only a little paint on the brush means you've more control over it.
Resist the temptation to push the whole length of the bristles into the paint in the belief that you'll have to do it less often because there's more paint on the brush. Not only does this make it harder to clean the paint out of the brush, but you're more likely to end up with too much paint in an area accidentally.
It's better to dip the brush into the paint frequently. It's far easier to add a little more paint to a stencil you're painting than it is to remove it without creating a mess.
The paint you're using for stenciling shouldn't be too liquid, nor your brush too wet (which thins the paint further), as the paint is then more likely to seep under the edge of the stencil, ruining the result.
