This infographic has a lot of interesting info, but the thing that is most valuable is the point about not using too many hashtags. Twitter recommends using no more than 2 hashtags in any tweet. Using more than that renders each subsequent hashtag less power and a more spam-like quality. No one likes spam!
So, keep it clean. Use hashtags for good and not evil. They are a powerful way to link up to conversations and even start your own. Remember, if you want something to catch on that redundancy is key.