For a professional Twitter username you may want to use your first and last name (perhaps separated by an underline). If it's for a business you can add a number to the end of it if the company name is being used. If you're looking for a more interesting nickname you can always browse Urban Dictionary to find some of the latest memes to enter the lexicon.
But what do you do if the name you want is already taken? TweetClaims is a very useful service that will constantly monitor Twitter to check if the name you want to register becomes available. Then you can get an email as soon as it's free!