Try the same lying on the floor - either face down or on your side. Same rules apply - knees together, keep the pelvis from rolling forwards (i.e. arching your back). If you're trying the stretch face down, it's a good idea to actively contract your abdominals - try pull your navel to spine - to keep the pelvis rotated back and stop the lower back arching.
A preliminary stretch that it might be worth trying is a kneeling lunge. This gives a quad/hip flexor stretch. Kneel on the floor (good idea to get a cushion or something to protect the back knee), then bring one leg out in front with the foot flat and the knee cap behind the heel - make sure you don't let the knee push forwards beyond the ankle during the exercise - basically the toes-ankle-kneecap arrangement needs to maintain a rightangle. Then slide the other leg backwards with the knee/shin/top of foot on the floor, until you've got a bit of stretch across the front of that leg. At this point concentrate on a upright posture in the upper body (I put both hands lightly on the forwards knee but down by your sides is fine also), actively contract the abdominals to flatten the lower back as much as possible (rotating the pelvis back enhances the stretch in the hip flexors and quads of the backwards leg), then slowly deepen the stretch by letting the forward leg bend further, lowering the groin towards the floor. If you want more of a stretch, you can roll the rear toes under or, only if you're really flexible, pick up the rear foot and bring the heel towards the bum - this is a really intense stretch. In which ever arrangement, hold for 20-30 seconds while breathing slowly - i.e. don't hold your breathe just 'cos you're contracting the abs. Then swop sides.
If your knnes are is good condition, just sitting kneeling down, bum on heels, will give your quads a good stretch.
