Are you trying to reduce the actual InDesign file size, or the Acrobat PDF, because they're too different things. You produce a PDF from InDesign, the resulting PDF is that size because of:
Quality of images used
Amount of images used
Compression of images
Types of images used (Raster or Vector/Complex Vectors)
Size of images
Fonts embeded/number of typefaces used (Subset or Entire glyph sets)
RGB output or CMYK output
There are many mitigating factors to the final size of a PDF, perhaps if you could clarify if you want to make the InDesign File Size smaller or end up with a Smaller PDF file size.
If it's the latter, what is the purpose of the PDF, is it for printing lithographically, digitally; or for screen viewing only?
I assume you're printing to Postscript then?
That doesn't tell me the purpose of your file though. Are you sending to a postscript RIP, or sending the PS through Distiller to make PDFs?
Are the PDFs for print or for onscreen?
Can't really advise you on how to proceed without knowing the purpose of what you're doing.
I feel it's all a bit quite vague.
A simple "Save As" will clear all Undo information that might have been building up over time.
If you *really positively* want to clean your file of anything suspicious, export to INX (or, using CS4, to IDML) and open that. Save a copy of your original file somewhere else first, as this is not a 100% error free guaranteed procedure.
If your file size is still beyond a reasonable limit: do you have any pictures embedded, rather than placed as a link? (Just Don't Do It.)
Do you have pictures with an extremely small (72 dpi) "Actual ppi" and a high (300 dpi) "Effective ppi"? That can make the preview images very large -- and these *are* included in your ID file.