Build profile first by stumbling VERY interesting webpages (with added reviews). If you reach 1000+ stumbles and 50+ reviews send private message to friend (power stumblers). Make a deal with him/her that you will stumble any webpage he/she will sent to you and he/she will do the same.
Website or blog owner usually accept this proposition. If rejected, unfriend him/her. besides there're 100s of thousands power stumblers out there. It's useless(and waste of time) to have a friend that didn't act as a friend.
It's better to have 20 power stumbler friends, than 200 average stumblers.
Being a power user in stumbleupon is easier than digg. And stumbleupon gives much better traffic that last 1-3 months. While digg on last a day.
As you can see I'm not a power stumbler....yet. But I study how stumbleupon works and how you will benefit from it.