i second the above post, regarding the PTS. Definitely, pick up the PTS and study it, highlight it, know it inside and out. that's exactly what to expect for the checkride.
as far as preparing for the training, study up for and take the two written tests. get those out of the way before beginning your ground sessions with an instructor. it's much easier to have the writtens done and out of the way before you go in to your one-on-one sessions with a cfi. that way, you can focus on the true instruction, not the faa-written test stuff.
the flying should be easy, just getting used to doing the commercial maneuvers from the right seat of your airplane of choice. also, get used to talking and explaining everything you're doing as you're doing it.
hardest part about the cfi is definitely sitting through the oral portion of the checkride. but, like anything, the more prepared you are for it, the easier it'll be. don't let anyone scare, intimidate or disuade you by listening to them tell horror stories of how bad the oral exam is. yeah, it's thorough and definitely a big event, but it has to be.. think about it, after you pass the checkride, you are now certified to teach people to not kill themselves or anyone else in an airplane.