When setting up your personal or corporate Twitter profile, there are several things you should keep in mind.
If the Twitter account is to extend your personal brand always use the combination of your first and last name when setting up the account (note you only have 15 characters for the Username and 20 for the Name fields). If it will not fit you may even consider adding your profession (i.e GaryK_Lawyer). This may even help gain additional followers that maybe looking for people in a certain profession. If your name does not fit within the allowed number of characters you may want to include your full name at the beginning of your One Line Bio.
I have found that the character limitations prohibit some companies from using their branded name within the Username field. This can be a challenge, but there always seems to be a way to provide a meaningful Username that conveys the corporate name. I would also recommend adding "The official account of COMPANY NAME." to the beginning of the One Line Bio. This can help separate your real company account from a "fake" one. Jeremiah Owyang, of Forrester Research, even goes as far as to recommend cross linking your Twitter account from your corporate website to bring additional credibility to the Twitter account.
Well what happens if your company name is already taken? There has been some companies that have successfully gained control by contacting Twitter support and pleading their case. Darren Rowse, of Problogger, suggests following the account name and then @reply to them to see if they are willing to transfer the account over.