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 -
http://www.feldenkrais.com/questel1.html
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 ;-)
http://www.nas.com/~richf/low4.htm
http://www.nas.com/~richf/low10.htm
http://www.nas.com/~richf/low28.htm
http://www.nas.com/~richf/low29.htm
http://www.nas.com/~richf/low38.htm