Andreh Anderson demonstrates a very useful sweep you can use when your opponent is trying to pass your closed guard with the gi.
Start this technique when your opponent attempts to open your closed guard with his knee in your tailbone. With your left hand, secure his right sleeve at the wrist. Your right hand will move to grab the gi pants of the knee that is against your tailbone.
With your grips secure, drop your left foot onto his right hip…
…and climb your right leg up under his left armpit. Use pressure to get his upper body to bend toward his right side.
Your goal is to try to fold him in half to his right by using your right leg to press into his armpit with as much pressure as you can generate. Your left foot on his hip blocks him from posting on that side and assists in folding him. Keep your grip on his sleeve as he starts to fall to his right. This will prevent him from having anything to base with on that side.
Once his back hits the mat, release the grip on his knee and shoot your right hand to the mat to help bring you over and to base in case he tries bridging you off immediately. Keep your grip on his sleeve with your other hand.
Finally, you have reached the mount.