When I graduated from art academy couple years ago, I noticed that without regular studies my drawing skill stopped in some point and progress get slowed, in my opinion I were just repeating some previously learned tricks and it was not enough for my new goals. I started to make drawings for our book, and it was necessary to make lots of quick and living sketches to catch new characters expressions. Then I had no opportunity to visit classes anymore, and that was first time when I seriously dived into studies from photos. In the college teachers didn't allowed us that kind of copying, only etudes from real models, they said it don't develop sense of volume. But I have no other ways to improve my skills, and I tried to do that, and surprised how it really helped me.
The key to making good studies is attention, photo must be interpreted not as mess of colorful spots, but as projections of real objects with all features of surface and lighting. May be it is simple and obvious thing, but only actual doing studies opens how important it is. Sometimes I just can't realize lots of small subtleties by just looking at an image, but copying opens them all. Other great thing is that brain begin to predict some features of realistic style after 2-3 days of studies and then even sketching from imagination becomes to look way better and detailed. And, of course, combitation of cool edges and soft areas matters a lot.