The best way to prep is for your instructor to give you a practice oral based on the way your examiner will do it. Those "Oral Test Guides" aren't really worth much any more. While there's a lot of good material in those "Oral Prep" books, the FAA guidance on practical tests no longer calls for those simple Q&A methods, but rather goes for "situationally based testing."
For example, instead of asking you what the VFR weather mins are in Class E airspace, the examiner may ask you whether or not today's weather is good enough to make your planned XC flight under VFR, and how you came to that determination. This will test, in a more realistic manner, not only your rote knowledge of 91.155, but also your ability to apply that knowledge to a practical situation, not to mention your ability to read and interpret all the available weather data, as well as determining from the sectional what airspace you'll be in so you know which paragraph of 91.155 applies along each part of your route. Those old-style Q&A books can't prepare you for that -- only a good instructor familiar with the concept of situationally based testing can.
So, for an IR test, the examiner may look at your flight plan, and ask you why the Alternate block is blank -- and have you justify that answer based on the regs and the weather. Or, if there is an airport there, ask you why you needed to put it there, and how you know it qualifies as a legal alternate today, which requires reference to weather, regs, NOTAMs, and the Terminal Procedures book. This is very different from the old days where an examiner would simply ask you "What's the required weather at your destination to not file an alternate? Can you use this approach as an alternate? What are the standard alternate minimums? Are the alternate minimums at this airport nonstandard?" and makes you think and analyze, not just parrot.
Now, there are still a few old-school examiners who pull out the Oral Test Prep book and start asking questions from it, but they are getting much fewer and farther between. Your instructor should be familiar with the testing styles of the local examiners, so s/he should be able to help you prepare, and give you a practice oral that reasonably accurately reflects that style.
