This is a quick read that will improve your game. You won't have to buy junk, and you won't have to change your swing. I am a believer that sometimes formal golf lessons are not worth the money, and that one can improve his or her game to the extent desired (and practical) without paying high rates for quick lessons.
Number 1: Ball Striking
Consistent ball striking is elusive. Golfers, when at the top of their game, are confident in their ability to strike the ball. This is one of my favorite drills on building confidence in ball striking at any level:
Go out to the range. Most ranges have flags from anywhere from 80 to 100 yards (I am banking on the assumption that your local golf range does too). Take aim at the pin and attempt to hit the flag (or the flagstick) using about a 6 iron and hit half swings focusing on making good sounds and hitting down on the ball. Continue until you succeed in hitting the flag or get extremely close to doing so. When you take a full swing it should result in a clean strike on the ball after doing this drill.
Advanced option: Attempt to hit the pin or flag with a small cut or draw. Once you succeed with this you can mix up the cut/draw/straight shots. Mastering this drill will work wonders with confidence and striking ability.
Pros: Increase ball striking, confidence, and proficiency of controlling spin.
Cons: You might run through your balls rather quickly. Get an XL bucket or visit a country club where the balls are unlimited if possible.
Number 2: Coordination
I've heard a couple of times, "did you know that Tiger Woods is primarily a left handed swinger?" I do not know exactly what this entails, or whether or not it is true, however my next drill usually increases both swing speed and accuracy. I picked it up from a Purdue Golf Camp a long long time ago.
Find an isolated corner of the range to ensure the safety of others and your dignity. Take a pitching wedge or close equivalent. Start hitting balls with your left hand only (right hand for lefties). Focus on striking the ball well, not distance. This will develop coordination and strength in all of the right places. Once you hit about three in a row solidly, try a normal swing. It will feel like the most control you've had over the club in awhile.
Advanced option: try it with only your right arm only as well. Your arms play independent roles in the normal golf swing, and the right hand usually is just there for support. However, control over the right arm is also essential to a proficient golf swing.
Pros: Increases strength, coordination, and confidence in the control of the swing.
Cons: can be embarrassing if you whiff the ball (you wouldn't be the first)
Number 3: Putting
Putting is what I like to call the key to the lowest round you've ever had. When the putts fall, the score falls at a seemingly fast rate. The key to making putts from any distance is being able to put the putt where you want to put it (and confidence to do so). In order to increase your chances of making long putts, or making pressure putts in an upcoming tourney, there is one simple drill that will prepare you for game time.
Go out to the course around twilight (presumably after a day's work of doing the other drills that I have mentioned). Mark a hole that has at least a little break to it. Grab about 4 balls and putt at that hole from 3-5 feet in random directions. Mix up the distance and the way that you are approaching the hole. Stay there until you have made 50 putts in a row. This may take awhile, however, persistence and determination are key. After you are finished, stay and putt from intermediate distances and finish all your putts (never pick up a ball no matter how close it is to the hole). This drill is better and more entertaining than just "practicing putting" by walking around the green.
Advanced option: You can make this drill as hard as you want to make it. Remember, this is just as much about building confidence as it is building accuracy. That is why the optimal distance is 3-5 feet.
Pros: Builds putting accuracy, confidence, and gives a golfer a sense of accomplishment.
Cons: Don't leave the wife or significant other waiting all night. There are other things besides golf.
Number 4: Sand Shots
Adding touch to sand shots is difficult. Sometimes it is difficult to adjust to different sands, too. On a side note, before a tournament I always recommend practicing in the sand of the course where the tournament is being held. Anyway, my next drill is potentially cruel. A long time ago we made our freshman in high school catch for us (you'll understand what I mean in a second).
For this drill you'll need a partner that is brave and not your first-born child. That, a net, or a good imagination. For all intents and purposes I will assume you have someone that is willing to take turns catching balls for you. Take a large amount of balls into the sand. Have your ballsy partner stand in front of a flag or target that is about 20-30 yards away, about where you want your ball to land. Hit the balls to where your partner can catch them. Do this until you are confident and your partner can work his or her way toward you. Ideally you want to do this until your partner can stand right against the lip of the trap and catch them with confidence. The added human element adds focus and confidence to your sand game. You will improve your sand game dramatically because of the natural effects of working with someone else. A net will work too, I just can't promise as quick of results.
Pros: Quick improvement, confidence, and building trust with a friend
Cons: Obvious, especially if you aren't a good golfer
Number 5: Balance
The TPI program focuses on balance, and for good reason. Full swings require balance to be consistent. There is a simple and easy way to achieve balance in your golf swing: placing your feet together on the range. Just take a normal stance, then put your feet together. Take medium length swings and then full length swings until you strike the ball well. You should be able to achieve distances that are almost as long as a regular swing once you're doing it right. After completing this drill you will be a better golfer for it. You might not be able to tell any major differences, but your body should have adjusted to make a more balanced backswing, weight shift, and follow through.
Advanced option: Lift your back leg and continue to make full swings. This is harder but also has a larger payoff in terms of your body adjusting itself to improve balance.
Pros: Better ball striking, consistency, and overall lower scores
Cons: Sometimes it can be uncomfortable if men put their legs together too tightly