Jane Friedman, of Writer's Digest fame, wrote an informative blog post a while back about how to create your Twitter persona to your best advantage as a writer and/or published author.
If you're new to the Twitter game, or just feel you could use a profile lift, she's got some excellent, basic advice. For example, if you're looking for representation, it's better to use small amount of space available to you for your bio to say what kind of project you're working on than to say 'I'm an aspiring novelist looking for an agent.'
She's got advice for your photo, too.
check it out here