How to keep your external HDD alive
Keeping an external HDD alive is pretty easy. Don't move or bump it while its running, never drop it (obviously). Always make sure that you safely remove the drive when you're done, don't just unplug it as that can corrupt files. Run a disk check every now and then. If you are on Linux I have some furthur tips but the above will cover you just fine on Windows and Mac.
You should also run a health check on the drive every now and then, just to be sure. A program called HDDScan on Windows will do that for you and it takes ~5 seconds to check.
Which one should I buy?
Each person will tell you something different but my money is on Western Digital drives. Always go for their Black or Red series, NEVER their Greens. Never buy Seagate, Toshiba or Hitachi if you can. There isn't really any specific features a drive has that you should look for. Oh, and if a drive uses its own encryption software NEVER use it. It is often unreliable and can wipe your data.
A big thanks to SK2 Anon for giving me this information!