I've recently begun playing again after several years away from the guitar. Posture concerns and aches & pains have been part of my daily routine over the last few months.
Like Kammes said, I've also found that taking a brief break before
I start to ache someplace is the best solution for good practice time. If I wait until something hurts, it's already too late and any subsequent exercises are wasted by attempting to compensate for discomfort and poor ergonomics.
I try and keep a mirror in the room I'm practicing in - for me at least, it helps immensely with correct posture. I don't have a video camera or else I'd tape myself for review.
The other thing I've had to come to grips with, is that not everyday is a "top form" day, at least for now. What I mean, is that I have days where I can play accurately and with expression, but the conscious effort is too much. Other days, I play to my best and I'm not really paying attention to what I'm doing with my hands, it just flows out. I try and mentally adjust to what kind of "day" I'm having, attempting to limit frustration that way.
One last thought, and something that has made a big difference in the short time I've been playing again - having a pre-determined practice routine is 1000 times better than not.