One thing that might work, if it's really cold outside and the normally-OK lock wont work, is go flic-your-bic and heat up the metal part of the key a little. Then quickly try it before it cools down - if the pins are frozen (they're small, so just a little dampness when frozen will make 'em stick) the hot key will thaw 'em out enough to work.

After that, maybe a little WD40 (That's what WD40 is used for - WD means Water Displacement - it's not really a lubricant.) might help get rid of the moisture.