Intense concentration
Some tasks require intense concentration for hours. One of the best rules we had was if you are still troubleshooting after an hour and not making progress then take a break; if you go two hours and not making progress then call for help; a fresh brain can see details that a fatigued brain can not.
Most of the problems I deal with are similar to minesweeper in that there are patterns that once you learn them then you can solve the problem very quickly. It also pays to involve all the senses in troubleshooting, listen for unusual sounds, look for something out of place, smell something burning and feel the movements.
The last thing is to take extra time if you are not sure if the solution will work or make the problem worse. If you are tired then you tend to want to end the efforts too quickly and if you are over confident then your assumptions will bite you.
