The absolute most important thing you can do for your health is be sure to have a least one Bowel Movement a day, two is better; feweress than one a day and you literally are poisoning your body, from the inside.
That said, There are a buttload (sorry, couldn't resist) of colon cleanses out there. Colonics are popular and expensive, and they remove alot of the healthy flora (bacteria) from your colon. This health baceria is what digests your food and leads to good overall health. probably the safest, cheapest, and most natural and effective colon cleanse is to mix a teaspoon of liquid Bentonite clay with a tablespoon of Psyllium Husk in a glass of water and drink that once or twice a day for a few weeks.
If you have not been eating your veggies for a few years, even an enema with a Fleet Enema from the drug store will clean an amazing amount of crap off the lining of your lower colon, (like the last foot and a half or so).
the best long-term solution is to eat sufficient raw or steamed vegetables. Done daily your colon will keep itself clean naturally, and keep everything moving along at a healthy pace.