During my time at university, I lost seven sets of keys. Seven. I once threw a pair away when I'd unpacked my shopping, it was that bad.
When I moved to London, my parents bought me one of those brilliant The Bloody Keys keyfobs and it lasted for - well. A while. A good while. I don't *think* I lost it - I think eventually it just wore away - but I honestly couldn't be sure.
These days I never lose my keys, partly out of fear (HOW much to replace this particular type?) but also because I religiously put them back in the same place. For a bit of extra help however, I love this Buckle Up key holder from thabto design. Just attached the keyring to your keys, attach the buckle to your wall, and slot them in when you get home. So you'll always know where your keys are. In theory, at least. What you do with them outside the house is anyone's guess.