Keeping extra copies of the video on your hard drive as back up is quickest, cheapest way to do this. But, video tends to take up a lot of space, so some other option is definitely in order.
The option I think to be the best is to put it on an external hard drive, and possibly another. Sure, you'll spend 60 to 100 or more dollars. But you'll find it to be the best way to organize content and protect the integrity and quality of the video.
I wouldn't reccomend flash drives. Many don't have adequete space to store a lot of video. Not to mention, a careless yank of the flash drive from your computer may wipe all the data.
You can also use CD, DVD, or even Blu-ray disc (if you have the rights means) and even tape. However, blank discs can be very tricky and evil. Some DVD discs will decay after a period of only 10 years. Your option then is to either purchase really high-quality blank discs and hope for the best, or print to tape and take very good care of the tape. If tapes are stored any place other some place cool, dark, and dry, you may have generations of the tape fade out. The only other problem with tape just might be that the technology to use tape will become scarce over the years.