I DID spend money on a commercial gym trainer and it was DEFINITELY a mess. I videoed myself from time to time so I knew I wasn't able to get to parallel without rounding my back - my trainer claimed I just needed to stretch my hamstring. After much stretching (and not much improvement), I fired him (and not just because he failed to fix my form...).
I started working on it on my own, but again, not much improvement since I didn't really know how to fix it. Then I saw mmgalb's log, where she talked about going to see Mike Robertson and get an assessment of her imbalances, etc. I emailed Mike (who is based in Indiana, I believe) to see if he knew anyone in the Los Angeles area (where I live) who could do something similar. He referred me to a friend of his, Craig Rasmussen, who works for Alwyn and Rachel Cosgrove's gym and I immediately set up an appointment.
It wasn't cheap, but after an hour-long assessment and watching a couple of my squat videos, he told me my hamstrings had nothing to do with my rounding - it was a stability issue, not a flexibility issue, and within 5 minutes, I was squatting well past parallel without rounding my back anymore. I now do a coaching session with him once a month and have him write my programs for me. Best thing that ever happened to my training.
Okay, so that was a bit of a novel, but basically I do think that if you're worried about your form, the best thing you can do is find someone who knows what they're talking about to help. Barring that, DEFINITELY video yourself and ideally watch the videos in between sets to see if there's anything you can focus on correcting.