I think everyone who has a lot of miles behind them has had your problem. I have replaced tires to solve the problem. Sometimes you will eventually find a cause. Try to always mount the first letter of the tire Name even with the valve.
Remove the tube and find the hole.
Then without removing the tire, lay the tube on the tire, valve by the first letter of the name and you can see where the puncture was made in the tire. That will be a clue for future flats. Good luck.