This is a challenge. Personally I never managed to do it without a heated bed. Many others have been successful and I believe the trick is to extrude at the lowest possible temp, and good bed prep.
If you haven't already, check out http://www.printrbottalk.com/http://www.printrbottalk.com/ where there is a very active community and tons of help.
I print on glass with Suave extreme hold hair spray (and heated bed). Ever since the addition of the heated bed my parts are flat every time, no more struggling, and when it cools a small tap and they pop right off.