Some good replies here that I wanted to add to.
The Feldenkrais Method is a system of somatic education - ie: learning how to use your body more efficiently. As a quick primer take a squiz at this -
I mention this because Moshe has an excellent series of lessons on achieving effortless ukemi. I can't for the life of me remember those (any feldie afficionadoes that can lend a hand?), so instead I recommend you try the following. You might be suprised at the results ;-)