What is the quality of the traffic from stumble upon? One of my sites gets about 80% of it's traffic from stumble upon but it's terrible traffic. Nobody goes past the front page and nobody registers. In my case, it's nice for vanity (higher visit count) but useless otherwise.
However, if you want to boost that traffic, the best way is with stumble upon paid advertising. I'm sure they will say it doesn't, but running a paid campaign greatly boosts your 'organic' stumble upon traffic, in my experience (I've done several experiments with several sites). Plus, it's really cheap.
