Choose a good handle
Twitter.com/mysquidgybum does not cut the mustard. The handle does not really help from a keyword rich/stuffing perspective however it does aid usability and CTR from the SERPS.
Equally ensure your account name cuts that same mustard.
Fluff the ego
Link back to other relevant people in your community, mention them (and then tell them) in Top X lists, etc that also aids…
To garner more mentions, retweets (mini tip: make sure you do not fill your 140 characters and allow for “RT @Username”) and in turn improve Twitter ranking impact establish strong relationships with relevant people.
Establish a quality profile
No one is 100% sure how twitter author quality is determined however it must include things such as presence of an avatar, number of followers, quality of followers, a complete bio, relevance to your area of tweeting, do not have a bunch of numbers in the username, etc.
As a way of measuring how your profile is publicly perceived use something like Klout
Google uses your twitter bio as its meta description hence optimise the 160 character limit (I never said I followed my own advice!)
Google displays tweets in the natural listings hence put your own site links in your posts to aid awareness. Also write keyword rich tweets and due to Twitters buzzy nature ensure you include posts about trending topics.
Hash Tags and Profile Mentions
Use them but do not overuse them (particularly hash tags) as you just look spammy.
Despite twitter links being rel=nofollow and thus in principle not carrying any link juice they do carry some weight; as confirmed by both Google and Bing (although the ‘SEO power boost’ does not last anywhere near as long as a ‘normal’ link), hence use this strategically in your Social SEO implementation.