This helped me, to quote Lyle
Squat stretch: Load a bar with a moderate weight. Get into your lowest squat position in good form. HOLD it and let the weight push you a little bit further down. That plus targeted isolated stretching will have the greatest impact.
I'm not going to the gym at the moment though, so I'll probably buy those elastic bands that allows to go into the squat position (don't have any weights, bar at home).