I am not used to use any site for increasing the twitter followers. But connection with the right people , providing them the interesting and new stuff in the discussion and maximum try to solve other's problem. As this will naturally be helpful to get the more followers. But the thing is to be live and to be responsive all time.
a friend of mine had great success recently using promoservices.webs.com
he says he paid for 12,000 followers and they gave him 17,000 and since
then, he's been getting alot of new followers each day..
Of course, before anyone even considers using a 'buy followers' site, no matter how the site in question tarts it up that's essentially the service most are offering, you should first consider that doing so is in breach of the Twitter terms and conditions and you face having your account suspended as a result. Hardly the best marketing strategy on the planet then...