Ergonomics are a challenge with breast ultrasound. The right breast is pretty simple and should not poser too many problems, but the left breast can be more of a challenge.
Firstly, I ask the patient to get really close my side of the bed before I start a left breast scan. I roll the patient 45 degrees (or whatever amount balances the breast) and use a triangular wedge to support the patient. If I am reaching, I tend to place my forearm on the patients abdomen to take some of the weight so I am not performing unsupported abduction. This technique works well for me and thankfully I have no injuries.