The moment you take your body out of a neutral, anatomical posture, you are reducing your body's ability to create movement efficiently. You are straining your lower back, and forcing your gluteals to work overtime to stabilize you so you don't fall over, causing pain in these areas. It will take some experimentation on your part to see which changes work best for you. I encourage you to do this experimentation with a friend or teacher, not with a client. That way you will not feel pressured to perform, but rather can really take the time to concentrate on your body and your reactions as you experiment.
Massage therapists tend to work with the client lying prone or supine on the table.
These client positions encourage the practitioner to bend over the client, especially if the table height is too low. Try instead to work with the patient in the side-lying position, which permits the practitioner to remain upright as she performs her techniques. The side-lying position can be the most comfortable for the client as well, especially clients with back or neck pain. You can also try changing your position relative to the client. For example, many massage therapists position themselves at the client's head to do long strokes down the back, especially at the beginning of a massage to spread oil and warm up the tissues.