I just took the GMAT yesterday, so I'll tell you what I can. (I'm assuming by pre-test and post-test that you mean the procedures on the day of the test.) Hopefully others will chime in to fill in the gaps.
mba.com profile:I had already completed mine before the test, so it was just a matter of confirming the info I'd already submitted and clicking through each screen. It's up to you, but it might not be a bad idea to go ahead and complete your profile before test day. Just one less thing to worry about.
Pre-test procedures:I walked in 30 minutes before my appointment time, and checked in with the guy at the front desk, who asked for my ID. He handed me a copy of the rules I had to agree to and pointed me to a chair where I could sit down and read them. After I had done that, I handed back the rules, and signed an electronic pad indicating I agreed with the rules. He then took a vein scan of the palm on both hands, and a digital photo. I was directed to the lockers on the wall where I should stow my personal belongings. After a minute or two wait, he escorted me and another test taker down the hall to the proctor. We were instructed by the proctor to turn our pockets inside out and demonstrate we weren't trying to hide anything (basically, we frisked ourselves; it was a little weird). She handed us our noteboards, pens, and a set of earplugs, asked if we had any questions, and escorted us to our computers. She signed me in, and off I went.
Post-test procedures:After I finished the test and saw my unofficial score on the computer, I raised my hand (as instructed to do). The proctor walked up, signed me out of the computer, took my noteboards and pens, and escorted me out of the testing room. She had me scan my palm one last time (you do this every time you enter and leave the testing room), and told me I was free to go. I walked out to the locker to retrieve my belongings. On the way, I stopped by the front desk, where the guy who checked me in had a printout of my unofficial score waiting for me. I picked up my stuff from the locker, and left the building. That was it.
I hope that helps!