You protect your photos the same way that you protect the rest of your data: you back it up. Backup strategy is determined by the state of cheap storage hardware.

There is no cheap hardware that will reliably archive your data, unattended, for 20 years. So avoid any strategy that involves the concept of "archive." Instead, leverage this useful property of disk drives: if you can successfully write some data to the drive, then you will likely be able to read data from any location on the same disk drive. If you do so on the same day, at least. With this principle in mind, you will need to place a copy of ALL of your data onto a disk drive that you use at least occasionally. As you use the drive, you are passively testing it.

First, upgrade your main computer disk drive. It must be big enough to hold ALL of the data that you wish to keep forever. Don't cheat yourself: if you want to keep certain photos, movies, music, etc. forever, it must all fit on the disk drive that you use every day.

Your main disk drive may fail, or you may alter its contents accidentally. Therefore, you need backup. This would be an external disk drive, your choice of USB, firewire, or eSATA.

Your external disk drive may fail, or the cheap backup software that you use might do something stupid to that disk. Therefore, you need two external disk drives, not one. You swap them each time you back up. Your goal is to have two external disk drives, both of which contain copies of all of your data.
Your house might burn down, so you need to cache a third external disk drive away from your house. Your job site might be the best location for this.

You can't depend on any of your disk drives if you don't test them. So, you need to periodically rotate your three external disk drives. Me, I take an external disk drive to work one morning a month. In the evening, I return with a different disk drive and start using it for backup.

Since you are taking disk drives out of the (relative) safety of your house, you should think about encrypting your external disks.

If you use this strategy, then you are protected as follows:
  - If your main, internal disk drive craps out, you have backups to recover from.
  - If one of your two in-use backup drives craps out, you will find out within a few days, when you try to back up to it.
  - If your house burns down, you have a copy of your data off-site.
  - If your offsite disk drive craps out, you will find out eventually, when you rotate it back to your house to use as a backup disk.

Conclusion: this is the minimum bill of materials for securing your data against single faults:
  - A big disk drive that you use for all of your day-to-day activities.
  - three external disk drives.
  - backup software, preferably with reasonable encryption.
  - a willingness to promptly replace any of the above items that fails.

It would be cool if my MacPro and my laptop used WiFi to sync up documents, preferences, media files and such. This problem is especially acute in iTunes where I have hundreds of GB of media on my main machine, but have to manually manage those things on my laptop. I wish Apple recognized this problem and solved it elegantly.