Tight gluteal muscles can contribute to a whole host of problems including low back pain, groin pain, iliotibial band syndrome, and knee pain among others. This is a nice relaxing stretch to do after a workout and cool down. Lie flat on a firm surface with both knees bent and feet flat on the floor. Cross one leg over the other, "guy-style", with the ankle resting on the knee of the other leg. Now bring the legs up toward your chest, grasping behind the knee of the crossed leg. You should feel a gentle pull in the buttock of the leg that is on top. In the photo here, the stretch is being performed on the right leg.
Keep your head and shoulders down and breathe and relax. If you feel any pain in the hip or back, BACK OFF! Don't stretch so hard. If you are very tight in your hips, you may need to use a towel looped around behind your knee and grasp the towel to pull the legs up toward your chest. Hold this position for 15-30 seconds, then do the same procedure on the other leg. Repeat as directed in the instructions from your coach. Keep your head and shoulders down, relax throughout the stretch. Crossing your legs "guy-style" stretches one group, you can get another stretch by crossing "girl-style" (knee resting on knee, rather than ankle on knee). Stretch only to the point of a pull never to the point of pain.