I try to wash my makeup brushes every two weeks or so, but sometimes that doesn't really work out (AKA most of the time). In between washings, I like to use Sephora's daily anti-bacterial cleaning spray. You can spray the bristles directly, but I spray it on a cotton round and whisk the brush over that. I like to see the makeup come off so I know it's getting clean. This stuff can be a bit drying on natural bristle brushes, so I'd use it sparingly on those.
