The softness and gentle give of a foam roller makes it a perfect prop for stretching. You can increase your range of motion in a hamstring stretch by propping your foot up on the roller [top image]. Or do the same thing with your arm to deepen this stretch for your lats [middle].
You can also use it as extra support in poses like this hip flexor stretch [bottom]. Slide the roller under the front of your thigh as you lunge forward with your opposite leg and sink your hips down the floor.
