Know the PTS. Private, Comm, and have the instrument stuff available too. Remember you can provide instrument instruction with an intial. The only things you can't do are sign off an IPC or practical exam. A student only needs 15 hours with a CFII.

As for the rest, know the essentials (wx mins, valid times for wx, charts, AF/D, aircraft systems etc) cold. Know where to find everything else. Be able to back your answers up with the FAA-approved literature. Advisory circular AC61-65 is your best friend. I believe the current edition of this gem is E.

Go to the checkride with a battle plan. Know what you are required to do per the CFI PTS and teach the examiner. Not the other way around. Engage him as if he/she is a zero-time pilot. Too many view this as an insult to their intelligence or worry about questioning their authority but you're this isn't your intention. You're demonstrating your ability to instruct.