Here are two simple drills we do every day to keep weight back, maintain a firm front leg and stay on the line of force when pitching.
From an open-hipped position (front foot toward the target/catcher), the pitcher does a snap drill while driving the back knee into the front knee. Then the pitcher takes a step back with the back leg. It almost has the sensation of "falling" back. The pitcher should fall back along the straight line of force. If the pitcher veers to either side, they are off-stride or off-balance.
Variation: Same drill, but when the pitcher drives the back knee into the front, she stays there balanced until the catcher throws the ball back.
Opening things up
From the mound the pitcher repeats the first drill, but with a full motion. In order to do this drill, the pitcher should be throwing at approximately 75 percent. This ensures that the pitcher isn’t straining too much to accomplish the goals of the drill.
Variation: Same as above, but from a pitching motion. I have to remind my pitchers, who are just now learning how to throw a rise ball, that lower body mechanics for a rise ball should not be carried over to a fast ball.