My midwife encourages crawling along the hallway, playing on the floor on all fours etc from 34 weeks. Before then, they just summersault around a lot (well 2nd babies do anyway). She also encourages leaning forward while watching tv and relaxing, sitting backwards on chairs etc...basically avoiding reclining on your back.

I did this religiously when pg with DD and if nothing else, it helped me feel I was doing something!