Lots of water, fruit, whole grains and fiber. Coffee gets things moving for me, too.
Apart from the few days after DS was born, I have never been constipated during pregnancy.
If you get constipated, use Colace to soften your stool. Personally, it took until day 6 after DS was born before I pooped, not sure when I started taking the Colace. This time I plan to take it right away, just to be on the safe side.