A widely known and frequently taught golf tip is to keep the left arm straight for as long as possible in the backswing. Although this thought is great for generating width, it's realistically a very difficult position to feel and to achieve.
So as an alternative thought, try this easy backswing drill from TG's Dan Frost. By placing a boxing glove on the back of the left hand, you'll be able to generate more leverage with a different feeling in the backswing.
Push the glove away from the chest, much like a punching motion. With this feeling, you'll achieve a much stronger backswing position that has width and is fully loaded, enabling you to drive into the ball with more power.
If your goal is to out-drive your playing partners every weekend, then this is the drill for you!