Storing previous versions?

I'm planning to use a wiki-style help system for my website. I've considered using existing open-source wiki systems, but it would be too much work to get them hooked up with my user system, etc. Like most wikis, previous versions of articles will be stored. I'm looking for ideas as to the best data structure for the previous versions (one table for articles, and another for each version of them?). Also, any ideas for having it not take up huge ammounts of disk space? Storing the difference between the next version and the old one, perhaps? -- The above message is encrypted with double rot13 encoding. Any unauthorized attempt to decrypt it will be prosecuted to the full extent of the law.