Jeff Bullas offers some excellent rules of Tweeting. I found the following two particularly useful:

Rule #2

Please, please don’t use more than 3 hashtags in a tweet. If you do use three hashtags, do it sparingly. It’s just annoying.

Rule #3

This rule is going to go against what most “tweeps” believe! Don’t strive for followers. Strive for engagement. Ask people questions and engage with them! They’ll like it. In the long run, you’ll get more followers this way.