I've had that problem with all my MBs up here in Canada during the winter. Especially after I wash the car.
I've tried many lubricants but none seem to work. I'll try the Boeshield if I can find it. I currently use a butane jet cigarette lighter that I carry with me (I don't smoke). I insert a 'dummy' key (4" slim metal rod)into my lock and heat the key FROM A DISTANCE with my lighter. Works like a charm. Don't know if I'd recommend this to anyone, though. Whatever you do, don't try and force the key! Usually one of the other locks will work - try the passenger or the trunk lock. Either should open your car if the locking system is similar to mine. My mechanic also recommended blowing out the lock with compressed air to get rid of the accumulated moisture. I've done this before and it seems to help.
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired
"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon