When choosing your Twitter username, go for something that is either your actual name or something that describes what you offer.
It depends which one you want to push more.
Do you want to create more profile for yourself or promote your cause? Are you looking to make yourself into a brand or make yourself the 'go to' person for one thing? In my case, I went with Matthew_Duggan (MatthewDuggan without the underscore was already taken).
I could have also have gone for something along the lines of LondonActor or SocialMediaGuy but when I set it up, I wanted to push my profile more than just my job title.
As an actor, I'm aware that my profile is quite important to my success in my chosen business, so I decided to go for a variation on my own name rather than say, LondonActor.
I also know that people buy people first and knowing my name is the first step of them getting to know me.
While you're deciding, let me give you a few tips on names:

• Go for something memorable, whether it's your name or what you do.
  
• Only use your business name if you want to create a brand for a large multi-national company on Twitter.
  If you're just on there to promote yourself and your small to medium business, using a corporate name will confuse people and most probably put them off following you.
  
• Don't use random or 'wacky' names like coolbeans1994 or luvbug_2143 (both names that are registered on Twitter at the time of writing), especially if you're not even going to bother filling out the biography information in your profile.
  It just looks odd and makes you really hard to find which can only be bad for business.
  
• If you don't use your own name, make your username something that people will think of when they think of your field of expertise e.
  g.
  : [http://www.
  twitter.
  com/MsInternetMkt] for internet marketing or [http://www.
  twitter.
  com/GardeningGuru] for gardening advice.