Ditto on the shoulder stretches... but you also need to move your spine. A great one is to stand, and then raise your arms overhead, palms facing. Then tighten your abs and glutes (your booty!) and slowly bend backwards, pressing hips forward.
If you can't do that bend standing, you can lie down on the floor, face down, and then with hands beside your shoulders, press up, keeping your hips on the floor. Bend back as far as you can. If you know yoga, this is a basic cobra pose.
Any time you are bending over a lot (gardening, etc.) it really pays to bend backwards every so often, to counter all the forward bending.