You can also diminish the damage after the dam has formed with...panty hose! Fill the leg of discarded pair of panty hose with a calcium chloride ice melter. Lay the hose onto the roof so it crosses the ice dam and overhangs the gutter. If necessary, use a long-handled garden rake or hoe to push it into position. The calcium chloride will eventually melt through the snow and ice and create a channel for water to flow down into the gutters or off the roof.
Next up, a list of long-term fixes that will help you get rid of your ice dams for good.