Good advice, but don't do as a friend of mine did, and attach them to the same keyring

I kid you not, he lost both sets at the same time

Luckily they were found when we sobered up.
I also had the pleasure of camping next to someone who had locked their spare keys in the glove box

Once they had managed to break in without doing too much damage, they had to have the alarm reset in order to drive the car.
And finally, once whilst on a Ski trip, a friend of mine lost his keys whilst packing the car for the return trip. It was snowing REALLY heavily (about 6 inches in 30 minutes) and he was desperate to leave for home and work the following day, so he had one of his employees fly out with the spare keys. The poor kid was still required at work the following day, even though he had been up all night helping out his boss

The keys were finally found in the gutter about 6 weeks later when the snow melted.