I’ve had several questions about this, recently.
If you cut-n-paste a long URL into your tweet on Twitter, it will automatically be shortened when you post the update. I think Twitter still uses the TinyURL service for this.
There are many other URL shortening services. All let you add a bookmarklet, in the bookmark bar of your web browser, which you can click at anytime to quickly create a short URL for the current web page.
I often use Bit.ly. Beside having an itty bity cute name, the service tracks basic statistics, like how many people click on each URL, when and where. This is helpful, if you want a sense of which posts are popular with your readers. Bit.ly also has built-in integration with Twitter, letting you post directly to your micro-blog, saving the extra step of shortening the URL on one web page, then moving to another to compose your micro-post.
One reason (of many) I use the Twitter application Twhirl for much of my micro-blogging is URL shortening is built-in, using whichever service(s) I prefer. Twhirl stores my Bit.ly login info, saving me yet another step.
