We just put together a presentation on social media for non profits that has some great examples of successful fundraising using Twitter, Facebook, YouTube, etc.
You can see it here: http://www.primalmedia.com/blog/social-media-non-profits
Great article, Heather - lots of specific, useful tips that are great for beginners venturing into Twitter.
The only one I slightly disagree with is #9 about limiting the amount of tweets you post a day to 5 or 6. If you are engaging with your followers, not just pushing content or asking for donations, you'll easily post more than 5 or 6 a day.
A recent TwitterGrader report from HubSpot showed that: "Users who tweet between 10 and 50 times per day have more followers on average than those that tweet more or less frequently." - http://blog.hubspot.com/blog/tabid/6307/bid/4594/Is-22-Tweets-Per-Day-the-Optimum.aspx
They suggested a sweet spot of 22 tweets per day, spread out over the course of a day.
Also, as Katrinah mentioned, using a tool like TweetDeck (http://www.tweetdeck.com) can make it easier to have conversations and really engage with your followers - building up the relationships nonprofits need to succeed with social media.