There are several great hubs on this topic. My favorite is http://hubpages.com/hub/Boosting-HubScores.
Here are a few factors that affect your hubscore:
1. Length of the hub, try to make it at least 600-800 words.
2. Add pictures and/or videos to your hub.
3. The content should be original.
4. The amount of traffic affects your hubscore. The more traffic your hub gets, the higher your score will be.