What's the best method to get to Pompeii for a day trip?

The train would be the easiest option in my opinion.

Take the train from Roma Termini to Napoli Centrale.
Inside Napoli Centrale you'll need to go downsairs and purchase a ticket for the private Circumvesuviana train line heading to an endpoint in Sorrento.
There are two Pompeii stops between Naples and Sorrento so be sure when you hop off the train you are at the Pompeii Scavi station. The ruins of Pompeii are right next to the station.
Naples to Pompeii costs around €2.90 one-way, journey around 40 minutes.

I do highly recommend a guide for the visit. The ruins are vast and sometimes hard to decipher. With a private guide you'll be able to understand much more of what your seeing, and you'll be certain to see the important bits. My one regret for our trip to Pompeii was not opting for a private guide!

If you only have 1 day to devote to this big trip and totally miss the beautiful seaside cliffs of Sorrento (very very close to Pompei) then just forget this trip and go to Ostia Antica instead.

That is not a fair response. It is long day - a couple hours of travel each way. But it can be done and many do it. While OA is certainly interesting and worth a visit. It is not a substitute for Pompeii.

Back2Italy: Perfect, thanks!

Tim: I understand what you're saying, but that was useless. I don't particularly care much for Pompeii, but my wife very much wants to see it. We're going. The end. lol I know Ostia is close, but I don't want to be rushed trying to work in that much of a journey into a day trip.

Does anybody have any recommendations for HOW to hire a guide at Pompeii?

There are official guides onsite, I think you can take a group tour or hire a private guide.

There were a handful of guides just before the entrance offering their services. We talked to a couple just to get an assessment of their English and accent. It was strictly by the hour. I think we hired someone for two hours and spent another two or three hours roaming around and going back for a more detail look at some of the places that the guide had mentioned. Cannot remember the price but thought it was well worth it. Unfortunately you really need to see the museum in Naples to put everything in perspective. We didn't get to the museum until a couple years later. One of these I think we will hit the museum in the morning and return to Pompeii in the afternoon.