Step 1: Apply de-icer to the lock in order to get rid of the ice and loosen the lock to ensure that you will be able to turn the key once you are able to insert it into the lock.
Step 2: If you do not have de-icer and are unable to find someone who does, light a match and apply it to the key until it gets hot enough so that the key can be inserted into the lock and be turned.
In my particular case, skipping directly to step 3 would be best:
Step 3: Check to make sure that the car you are trying to get into is indeed your car. After all, won't you feel stupid spending close to half an hour trying to get into someone else's car.
(It's because of posts like this that I blog anonymously.)