A buy local group from Grand Rapids has done a very thorough study on the impacts of buying local in their community. You can read the report (PDF) here. Or you can visit http://civiceconomics.com/localworks/.

According to the study a 10% shift in the market share of local independents would produce:
  + Nearly $140 Million in new economic activity,
  + Over 1600 new jobs, and
  + Over $50 Million in new wages

As a local consumer a simple shift of 10% of your purchases to a locally owned business can produce the above effects!