A great way to stretch your hamstrings safely and avoid your belly at the same time is in a supine or reclined position. Lay on your back with your left leg stretched out long on the floor and your right leg straight up toward the ceiling.
Place a strap around the ball of you right foot. With your arms straight and your shoulders coming down toward the floor slowly bring your leg toward your chest.
Work your hands up the the strap as your leg gets closer to your torso so that you can keep your arms straight and your shoulders relaxed.
If you find that your belly is inhibiting your ability to bring your leg in close focus on bringing it along the side of your body.
Encourage your lower belly to sink into the floor as your leg comes in to your chest creating a hollow belly and more space to stretch your hamstrings.
Once you have reached your maximum press the sitting bone on your right leg down into the floor to increase the length in the right leg a bit more. Switch sides after several deep breaths.