You probably have weak shocks, and possibly other worn suspension components. It is the job of the shocks or struts to absorb or dampen the feel of the movement of the vehicle as it goes over uneven surfaces. As the struts/shocks wear, you will feel it more and more gradually over time. It will also cause uneven wear on your tires and reduces the handling ability of the vehicle.
I would take it in to a certified technician and have them check it out for you.
〉 Answered on May 13th, 2009 by Bogi Lateiner, Owner and Technician at 180 Degrees Automotive