JeSuisButterfly's suggestions to drink adequate water is an easy way to get started. And fresh squeezed grapefruit juice helps a lot of people too, just don't mix Sugar with it. It seems to have a stimulating effect on the colon. Try to drink at least 8 ounces of it in the morning.
Also, I believe that a properly administered cleansing enema is the quickest and most natural means of achieving a bowel movement when you are constipated. See the following links for more information: