If you are having problems this winter, purchase a roof rake to get the snow off the roof. You don't have to go all the way up, just try about half way or so. In the spring time, you should redo your roof and have the Grace Water and Ice Shield underlayment installed directly on the roof deck. My father used to break his back trying to rake it off and then when the dams did form, he basically ruined the roof by hitting it with a hammer.
Two websites: The first one is your solution and the other one is a local newspaper article about ice dams and some solutions.
http://www.graceathome.com/pages/roofing.htm
http://www.citizen.com/apps/pbcs.dll...WS02/703059790