It is customary to thank those people who retweet your tweets on Twitter. Twitter can even email you to leave you know who retweeted you. Thanking someone for doing this builds Twitter community connections.
Here is an example of what I have tweeted to thank my retweeters:
Thanks for the retweets: @JoeBRealEstate @getrealresults @Mortgagespei @Mortgage_Muscle @MarkBall_LMI_UK Here's to a great week of business
— Jeff Brown (@4JeffBrown) October 28, 2012
Depending on how many people you have to thank, you can do it one at a time for only a few, or you can do it a bunch at a time if there are many to thank. Either way, make it a priority to recognize those who help spread your tweets further out into the Twitterverse. As a business this should be common practice.