Another option nobody has mentioned. You said you're using the on camera Flash. I assume you're saying that the flash drains the batteries on the camera pretty quick? If the oncamera flash is sufficient for your purposes, what about just getting an adapter to run your camera off of AC power? You still have the flash recycle time but if you're not firing at full power I don't see that being much of an issue.
Canon 5D Mark II, 50D, XSi, 24-105L IS, Sigma 35 1.4, Canon 40 2.8, Canon 35 2.0, Sigma 10-20, Tamron 17-50, Canon 50 1.8, 580 EXII, 430 EXII