Know 91.167, .171, .173, .175, .183, .185 by heart. Know when you're required to have an instrument rating. Know your charts (what ever kind you use NOS/Jepps) cold...Pitot/static and Vacuum systems and their errors...magnetic compass errors/how to use it. Service volumes of VORs. What is: MEA, MRA, MAA, MOCA, OROCA, etc. Where do Victor airways start/end? Jet Routes? O2 requirements and illusions. Weather: Be able to read the charts, winds aloft, METAR, TAF, etc. What causes ice to form? How can you avoid that? What do you do if you end up in icing conditions?
My instrument oral wasn't all that bad...I had to plan a cross country flight in front of the examiner (he called me 10 minutes before and asked me "do you want to do your checkride now?" so ... no time to prep). Then he asked me about weather...why I chose the routes, altitudes, airways, etc. Asked me about ICE...a lot (it was November). Talked about pitot/static, vacuum, VOR services, NDBs and how they worked, RMI, HSI, OBS, asked me if I wanted to go fly since it was relatively low IFR...said yes...an ILS, a Back Course, a hold and a LOC later and I was done.
Good luck! Hope this helps. If you weren't ready your CFI wouldn't sign you off...just do what you do with him/her with you.
-mini