Efficient string concatenation? -- For generating HTML pages (mod_plsql)

I have been developing simple, small web apps in the past where it's pretty much safe to just call htp.p() as you go along through your application logic.

But as my applications become more complex I am realizing that it would be cleaner to accumulate HTML output into an internal buffer before finally "printing" the page to the web browser at the end.  This is proving to be important because if I encounter some kind of error or exceptional condition near the end of the application logic flow, it would be better to construct a completely different page (an error page for example) than to start out sending a positive result page and then interrupt the page halfway down with an error message).