Print your registration number on the drone
Once you've registered you need to legibly print your new registration number somewhere on the craft. If you don't feel like marking up your spankin'-new toy with a Sharpie, scrawl the number on a sticker, or even write it on a piece of paper hidden inside the battery compartment. If you're knee-deep in drones, you only need to register one -- but make sure the number is on each of them.
Keep your registration with you when you fly
If law enforcement ever asks you for proof you're allowed to fly your drone, you'll need to produce a registration form. However, you don't need to print out a hard copy and carry it wherever you go, since an electronic version pulled up on your phone via the FAA website works just as well.
And re-up every three years
You're fine to fly for a few years once you register, but you need to renew every 36 months, which'll cost you a cool $5 each time.