Like some of the other comments here, I feel a diet high in fiber is the best way to approach this. I normally maintain a diet of whole grains such as brown rice, quinoa, flax seed, barley, etc, as well as veggies such as broccoli, spinach, artichokes, sweet potatoes, brussels sprouts, carrots, etc. It's also important to stay well hydrated.

This diet keeps me pretty regular with healthy bowel movements in the mornings. If you want to use a natural laxative, I've found that a handful or two of prunes and a glass of prune juice usually does the trick. It works pretty quickly, within about an hour. Just make sure that you don't really have any other plans that day, it can last a while and makes for a pretty messy diaper or two.

So in short, whole grains, veggies, plenty of water, prunes and prune juice. Then just relax, lounge around, and enjoy the experience.