this happened pretty frequently back at school. the bike and lock lived indoors, so when they went out and got locked up, everything would thaw, then freeze around it. i flushed my lock out really well, and made sure it was cold and dry. it helps if there's a place outside to keep the lock (a long ride with a cold peice of chain really chills your body though). when locking, put the opening facing down. stretch a little bit of old tubing around it for slushy times, or find some litter (mcdonald's cup) and wrap it up. duct tape too.