agreed with what others said RE: fix air leaks.
2 more things you should consider: (1) raise the "R" value on your attic insulation to as high as possible, anything over R48 would be highly desirable; (b) revise your roofing ventilation with additional roof vents and enough soffit vents to rid of the trapped air (most likely warmer air in the attic when compared to the outside) to promote proper air circulation, moisture and temperature equalisation.
This is what I used to perform calculation on my roof ventilation:
http://www.airvent.com/pdf/literature/PAVbooklet.pdf
Once you have all of these done, you shouldn't have to worry about ice dam anymore.
Q-TD