If standing, this is usually about hip to waist level. If you need to perform tasks at a low level say for a bath or bed, kneel to work rather than bending at the waist. If you have to stand for long periods, put one foot up on a footrest (or a couple of phone books) to relieve the curve in the lower back. Swap feet occasionally. If you are cleaning, put the vacuum or mop in one hand while putting the opposite leg forward to work, with the forward knee slightly bent.
Also never twist or turn to pick something up. Face the object you intend to lift. Avoid jerky sudden movements. When possible let someone else lift heavy objects.
Support your lower back with a pillow in the small of your back. Sit well back on the chair having your thighs fully supported. If you are unable to rest your feet comfortably flat on the floor, use a footrest or something similar to elevate your feet. It is ideal if your knees can be at a higher level than your hips. Some women will rest their legs out on another chair. Have your chair at a comfortable distance from where you are working so you do not have to lean forward.
If you are sitting to rest, avoid slumping down. This strains your neck and shoulders. Be aware when driving of having the car seat forward as much as your belly will allow (wear your seatbelt). When getting out of the car never kick the car door open with your foot. This can twist your lower back.
A firm but soft bed is ideal. Some women place a flat pillow or a rolled up towel under their pregnant belly for support. When getting up from a lying position turn onto your side, push your self up to a sitting position with your hands and then stand.
Sitting straight up from a back lying position normally requires using your abdominal muscles. In pregnancy these muscles are over stretched and weakened. This gives you a tendency to overcompensate, using your back muscles and straining them.
Some people use the Alexander Technique, or other disciplines.