as somebody else suggested you can use mag citrate. It usually works within five or six hours. Get it really cold in the fridge and drink it with a straw so it doesn't taste as bad.

Or you can use a fleet's enema.

Both of those are short term options not for frequent use. Once you get cleaned out you can try some of the other options people have mentioned in this thread. Fiber and drinking plenty of water should be routine. Activia yogurt has helped me.