I know a few ways of escaping yoko shiho gatame. But only one is easy to pull off. When he grabs you, you should push his head with your free hand underneath the leg where his arm is and you have a triangle.
The other way is very difficult if your opponent is even a bit heavier than you. Say the he has your right side. Grab his belt with your left hand and pull just a bit so you can create the space to place your elbow on his stomach. Now bridge, pull with your left, push with your right and flip him over you so he lands on his back.
"He who defeats himself is the mightiest warrior" - Confucius(I think)