i agree best thing is prevention, but i never once reclined in a chair, and lay on my left side from about 5 months.. but had an insane posterior labour.. some pelvis' just swing that way LOL.. I found the shower to be extremly helpful and also dancing with my midwife(swaying my hips thru the contraction).
Practise things that make u comfortable now- like the ball and all fours and beanbags etc...
Not all posterior labours are the same.... but great to know all ur options.