One of the best videos I've watched for technique is the one below, on performing a barbell squat. The lesson starts at 1:20.
After watching this video, I took nearly all of the advice, and managed to improve my 5 rep squat weight by 25%. It's still not a ton of weight, but it's more than I've ever done. All it took was some good advice on technique, losing the sneakers, and a $15 weightlifting belt!