Do you hit "over the top"? Does your swing move from outside to inside? Do others tell you that you swing too hard? On the range, do you hit maybe one out of 10 balls far and straight?
Did you know that all this happens because you are using just your arms? In order to hit the golf ball well, you have to use your hips and legs, not your arms. Think of your arms as simple attachments to the club. Other than that, your arms do nothing to give your swing power. Nothing at all!
I've tried a lot of swing fixes over the years, but the one I keep coming back to involves imagining that you are swing in an circular arc. In other words, on the back swing, imagine your club is moving clockwise from 12 o'clock to 3 o'clock or 90 degrees(if you are a leftie, then it's 12 o'clock to 9 o'clock). Once you get to 3 o'clock, rotate your hips back to their original position, keeping the club back the whole time.
When you are taking your back swing, be sure to rotate your shoulders only, not your hips. This shoulder rotation will build up a lot of power that you will release later. Think of your body as a fulcrum. Rotate your shoulders around this fulcrum to 3 o'clock.
On the downswing, rotate your hips and the club will follow naturally. Now you are letting your large leg and hip muscles move through the hitting area first. The club will naturally come second, as it is attached to your arms. You will release your built up power at this point and the club will make excellent contact with the ball. All this without any extra effort or help on your part.
On the driving range or at home, practice this swing in slow motion. Once you rotate your club back in a nice arc, rotate your hips back to their original forward-facing position and stop. Take a look back at your club. Is the club bisecting your body? In other words, is the club laying fairly flat? It will be if you have not moved your arms forward.
Now start again, taking another swing in slow motion. After you take the club back to 3 o'clock and then rotate your hips, look back at your club again. If it is bisecting your body, then start to move your hips and legs through the hitting area. If you do this in slow motion, you should notice that the arc the club is taking back to the ball is the same(or nearly so) as the arc you made on the back swing. Now you are moving from 3 o'clock back to 12 o'clock. Notice that you have not consciously moved the club with your arms; it simply follows your hips and legs through the ball.
Once you pass through the hitting area, you want to swing in an arc that moves from 12 o'clock to 9 o'clock(or 12 o'clock to 3 o'clock if you are a leftie). Moving through the hitting area in an arc will help your body turn through the ball and make a good follow through. If you do all this right, you should be able to watch the ball for about five seconds until it comes down.
Here's something else you can do that might help you with this type of swing. Imagine you hitting a baseball with a bat. This is that type of swing, flat and moving back 90 degrees, forward 90 degrees, and again forward 90 degrees on your follow through. As you take the club back 90 degrees with your shoulders, you are storing up a lot of power. When you rotate your hips back to their original position, the club will start to come down with its own momentum and more than adequate swing speed to make the ball go far and straight.
This is the only type of swing that works for me consistently. I'm a total amateur and certainly no expert on the golf swing. This swing may or may not work for you. I've taken many lessons, and the above is what I've heard over and over in different ways. Now I'm finally starting to do it! And finally starting to enjoy the game of golf!
If this swing does not work for you, then I'd recommend you try The Simple Swing. It is an illustrated ebook written by a teaching professional. I bought his first edition back in 2002 and just recently bought his newest edition. It will cost you $47, but it is well worth it.