I created an 8-page booklet in InDesign. I used the Print Booklet feature to create a .ps file. The preview looked correct but the Acrobat file was incomplete, with only 4 of the 8 pages imposed ie: 2 spreads and they were not correct.
What's the trick to making a print ready PDF booklet?
No trick. If Print booklet didn't work it's almost a certainty that you didn't set something correctly (you need to go to the second-level setup dialog to enter page parameters).
For Mac users (Windows, too), for simple 2-up saddle-stitch booklets, though, there are two free scripts from Dave Saunders that will impose a copy of your file that you can then export to PDF -- no postscript required: