You've paid a lot of money for your Disney vacation and then, you lose the key to the whole thing: Your park tickets. What do you do?
Well, if you're a resort guest and you've had Disney put your park ticket on your room key, it's as simple as going to Guest Relations (located inside every theme park) or the concierge desk at any resort and having it replaced. Thankfully, all your information is in Disney's system, so you'll get a ticket back that has exactly the same information on it as the one you lost. This is why I always have Disney put my tickets on my room key, even if I don't buy the ticket through Disney and use a service like Undercover Tourist instead. Keep in mind that Disney will only add tickets they sell or an authorized dealer's tickets to your room key. If you buy tickets off site from discount brokers not affiliated with Disney, it's highly unlikely they will add it to your room key.
If you're staying off site, it can be a little trickier. A cast member friend of mine who works in Guest Relations recommends always making copies of your tickets before you go to Disney. I actually make three. I put one copy in my luggage, one in my handbag, and I leave one at home. It's probably overkill, but it gives me peace of mind knowing that worst case scenario, there's a copy of my ticket somewhere. Your travel agent and reputable vendors will also keep records of the tickets they sell and work with the park to reissue another ticket, but this can be very time-consuming. If you have your ticket information, all you have to do is go to Guest Relations and have it replaced. Disney isn't technically required to replace lost or stolen tickets, but they always do so with proper documentation.
Finally, if you discover that you've lost a ticket with a non-expiration option on it when you get home, you can send your inforation to WDW.Ticket.email@example.com to get a replacement ticket.
ETA: Thanks to Julie for pointing out that you can also take a digital photo of your ticket. It saved her trip!