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If your server supports it, the Server Side Includes (SSI) is the best way to go. A search should turn up lots of info.
If you can't use SSI, then importing HTML via a document.writeln JavaScript works well also, in fact faster - but a small % of users may have JS disabled.
Either way, you just need to maintain/update one file
Sorry for getting out of the subject here.
But i would also suggest using SSI, it's a very easy way to change information.
And on my sites about 97% of the users support Javascript (if you are going to use keyplyr's solution).
I "flattened" a framed site for a client about two years ago. I was somewhat concerned with changing the user experience, so I didn't change the appearance very much. Just that now the nav section and header scrolled away, instead of staying fixed. I also retained the page filenames I had used for the content frame, to avoid 404's on the search engine listings that existed.
It was really good to see the response in the actual stats. Sales went way up within a few months (improved SE rank helped that I'm sure) but the most important stat for me was page views per unique, which more than doubled almost immediately. The average user was clearly hanging around longer and looking at more.
I've said this before, but frames just don't make sense to the entire online population. For a certain subsection (my guess would be the technically and analytically oriented) they seem like the best thing going. But others (and a significant number of others) are just mystified.
If you get really creative, you can create a single page template and a database that drives it. Then you can just copy and paste your old site's info into the database, and away you go. New pages can be entered in an interface in the backend.
But it all depends on tech level and most importantly, budget!