After coming close to being unable to pay at a restaurant once, I do this with money, and it works well. It's not cheap in the same way, so I do have to only put it in places where I'll remember to retrieve it later (usually just in an inner pocket of each of my frequently-used bags). But, having extra money with me that doesn't go into my "do I have enough for this outing" calculation has saved me some worrying.

Actually, I guess this is a general strategy for stuff you might unexpectedly need, or might loose the first copy of. I've also done it with travel documents, (non-perishable) snacks, medicine, and a few other things. Usually I just put a couple copies in each purse or backpack, though; I haven't tried many creative hiding places.