Wow thanks for all the great info everybody. Good point made by people suggesting that I invest in more storage space as it is not that expensive. Thanks for all the other tips and bits of info as well.
Don't forget to make proper backups. A good backup strategy is at least one extra local copy, and then another copy backed up somewhere geographically separate. Some of the cloud backup services are good. I personally use CrashPlan, but there are many others that are good as well.
EDIT: In this case, proper backup is the RAW/DNG files, plus any Lightroom catalog files. Remember you don't need to backup the Lightroom catalog caches, just the .lrcat file.