Prunes are really good for it
Flax seed oil capsules work well too with the benefit of being good for your skin as well :)
Drink more water (I know, I know) and you can safely take a 100mg colace capsule (generic name is docusate sodium) once or twice a day. It's a stool softener not a stimulant or a laxative so it won't MAKE you go and won't stress out your almost-ready baby, either.
Also try putting your feet up on a low stepstool while you're sitting on the toilet. It puts you in more of a squatting rather than a sitting position which can make it much easier to go.
Dried prunes also really help
Spinach cooked, bananas and plenty of water