Fred J. Miller offers camps during the summer all over the country - you can find out more here: http://www.fjmclinics.com/
Also, sometimes local guards/bands/whatnot will have clinic/camp days over the summer, just take a look at their site. I know there is a thing like that with a few corps (Glassmen and Cavs I know for sure), and Michigan State University has a Performing Arts Camp every summer for guard and drum majors.
Hope you find something good!