There are a number of free tools on the web that will convert a long URL to a short one that you can use for your Tweets on Twitter. Albion Web recommends using bit.ly. The search engines like it, we like it and once you start using it we're sure you will as well.

It doesn't just shorten your URLs it also shows Traffic, Conversations, and History. You can also share your bit.ly link in Emails, Facebook, Blogs - in other words anywhere you want to give someone a link and see whether they've actually clicked through on it.