John Haydon is a Boston-area social media marketing guru. He has one of the best websites I’ve seen that gives a step by step of tools, tips, and suggestions that you can use to dramatically improve your Twitter knowledge, and thus your Twitter presence. The goal of his website is to help nonprofits bring in more funds. I think if you read through his various tips, you’ll be fulfilling his goal.
http://www.corporatedollar.org/resources-twitter-folks/
And if you want to follow John on Twitter, his Twitter name is @johnhaydon
NCE Social Media 
Twitter For Marketing – This site also has great information. It includes tutorials on twitter and using twitter tools, and also the best way to use twitter for marketing. It is definatly worth a read.