If you have the equipment to do so, try tumbling UP an incline (cheese wedge.) If its large enough - back handsprings, but if its small, simply doing round-offs with your hands up the incline, with feet landing off the tall end, will make you have to drive your back heel harder, and push through your front leg more to get over. When you return to the floor, doing in on a flat surface will feel easy and more explosive.
If you do end up working you BHS up an incline, I would recommend getting a spot the first time. The first one WILL feel weird, and will require you to get your arms up quickly, and snap aggressively, or you will end up doing a back-face-spring.
If your really advanced, you can start at the flat end of the wedge doing a round-off away from the mat. Then mark the spot you land the round-off. From there round-off towards the mat, (your feet should land where you started your round-off before) and add a back-handspring up the incline.
Best of luck on your tumbling,
Coach Matt
Full-Out: Cheer and Fitness