Ideally, the best swing thought is no thought at all. When you are playing at your best, you are playing on instinct. When you are in the zone, the fastest way to lose it is to start thinking. When you make that occasional great shot, it more than likely came out of the blue with very little if any thinking about what you were doing. Then, you end up confusing yourself by trying to think what you just did that allowed you to make such a great shot.
The time to think about your mechanics is when you are practicing. When you play golf, it is time to perform on instinct without thinking about your swing. As a matter of fact, part of your practice time should be spent hitting balls without any swing thoughts to get use to playing on instinct.
So how do you train or condition your mind not to think when you set up to putt, chip or hit the ball? Believe it or not, it is just as easy as training or conditioning your body to do 100 push-ups. If I would give you ? 100,000,000 (that's right one hundred million Euros) as soon as you could do 100 push-ups, how long would it take you to train your body to do 100 push-ups? If you were in good physical shape, it wouldn't take you too long. If you are out of shape, you could probably do it within a year. But, you could do it, if your desire was strong enough and you are physically able.
What amazes me is to hear a golfer tell me that it is hard to still the mind and not think. It is only hard if your mind is not properly trained. You can't read a book and expect to be able to do 100 push-ups without exercising. So why does a golfer expect to be able to stop his thoughts without the proper mental training?
Just like it takes time and effort to build a strong body, it takes time and effort to build a strong mind that can clear itself of all superfluous thoughts when you demand peak performance from your body.
Before every shot you attempt train your mind to focus on your target with total positive expectations of being able to make the shot. The only thing left to do is to step up to the ball, go through your pre-shot routine without any thoughts except focusing on your target and hit the ball.
With practice you will be pleased at how well you will be able to still your mind and how many more shots you will be able to make. Focusing on your target and not thinking is just as easy as doing 100 push-ups. All it takes is the time and effort to train your mind once you know how.
P.S.: Do you feel nervous on the first tee? Do you feel under pressure sometimes when putting to win? Are shots over water a problem for you? Then this problem can be a mental one. Here is a program that I can recommend to help you overcome it: The mental side of golf