Lying on my side flat takes the most pressure off I guess, but I do prop myself up a lot. Standing up is the worst, sitting up straight not much better, but it feels like if I lean back enough to tilt my pelvis its pretty much ok, but the best position is lying down on my left side like when I sleep at night. If you really need to take pressure off in the hospital they will put you in "trendelenberg" position which is a fancy way to say slightly tilted head down. If you have any way to do that for a little while each day it may give you a rest. Tilting the sofa slightly by putting blocks under two legs, or the foot of your bed up would work. Don't stay that way too long though, it will give you a headache.
I know someone who is a serious yoga practitioner and she regularly stood on her head during her pregnancy to stop pre-term labor contractions. No clue if that would work for anyone else but she insisted it worked for her. I'd ask my midwife before trying it, and dont even chance it if you aren't supremely balanced while standing on your head!