I have the same problem. What I've found really useful is this simple exercise: hang from the pull up bar (like you do when preparing for pull-ups), lift both of your legs until they make 90 degree angle with your back, then slowly lower them down; repeat as many times as you can, but 10-20 repetitions daily after work are usually enough for me. Another simple exercise is a little less effective, but doesn't require any equipment: stand upright your back, shoulders and heels touching the wall, extend your hands forward then do normal squats.