One of the quickest ways to improve is to practice shots you don't normally hit. If you normally play a cut, can you play a draw? Can you hit a low drive and keep it straight, or a high drive? Can you knock down a 5-iron?
Nobody knows your game better than your favorite opponent. Here's a little game you can play to improve your game and try new things at the same time. Set up a match with your pal and call each other's shots. If your pal normally hits a draw, ask him to hit a low cut. If he likes to fly the ball back to the hole, ask him to run the shot up. He'll be doing the same to you.
Keep it friendly and don't get too nutty - asking for a 60-yard slice or a 7I from the tee on a par 5 isn't "fair." Keep your own score, and if you want to bet, mix in a penalty shot for failing to hit the called shot.