If I lose a Mercedes smart key fob, how do I get a replacement? Is there anything I should do to prevent the old key from being used if it was stolen?
You're going to need a new key, as used, already programmed keys will not work. You'll have to get your dealer to order the key as they're not keys your dealer or mechanic can make. You'll need to know your Vehicle Identification Number (VIN) and bring two forms of ID (in my experience) to get a new key; they're very picky about giving out new blank keys, so you need to make sure you have the proper information and go to an authorized Mercedes Benz dealer.
When talking with your shop, make sure you tell them you lost your key as you probably need a Replacement key. Replacement keys override previous keys which means your other key won't work if you find it again. This is important because if someone stole your last key, they can open and start your car freely until your car has a replacement key programmed.
If you think you might find the key again you can also have the mechanics disable the old key's code with the Electronic Ignition Switch (EIS) system; they need special hardware to do this so you can't disable the key yourself.
Replacement keys take much longer to program (around 90 minutes or more) than Additional keys which work right out of the box; you can plug in an Additional key matching your VIN and the key should work right after you plug it into the ignition. Because of this you might want to just disable your past key and get an Additional key if you need to use your car as soon as possible.