One thing that has worked for me is to have your current boot camp clients recruit for you. I offer discounts for every person they bring and even free camps if they bring enough participants.
Also I try encourage them to put a note on face book after class, just seeing their post keeps others interested and some inquire about the camp.
The other thing that has been successful is to always wear a t-shirt promoting your brand and have other wear the shirt, it is free advertising after the initial cost.
Lastly always be working on increasing your contact email list and stay in contact often.