Stick a golf towel or hand towel under your right arm during the swing. If the towel falls out from under your arm, you are not swinging on the correct path.
Stand a little taller than normal when you address the ball to eliminate a wide swing plane and give you a greater chance to come underneath the ball and hit it straight.
Make sure your grip is correct. If your grip is not tight enough the club head is not solid as you hit the ball. Don't over tighten your grip or you will prevent the club from swinging on the natural path.
Don't break your wrists until they are directly over the ball or you will release the club too early and cause it to strike the ball from the outside.