In "Contents of the Dead Man's Pocket," what mental strategy does Tom use to get himself to move along the ledge?

In the story, Tom uses one main strategy to keep himself moving, and that is blocking out his own thoughts. He forces himself to concentrate on mechanical movements rather than allowing himself to truly think about what's going on. Of course, that strategy fails a lot, but he clings to it. Let's take a look at this strategy and the others he uses during his harrowing excursion on the ledge of the building:

In the back of his mind he knew he'd better hurry and get this over with before he thought too much, and at the window he didn't allow himself to hesitate.

Above, you can see that he's already steeling himself for the task by telling himself not to think.

It was hard to take the first shuffling sideways step then--to make himself move--and the fear stirred in his stomach, but he did it, again by not allowing himself time to think.

A few moments later, Tom keeps on blocking out his thoughts by focusing on quick action instead. He continues this process:

He simply did not permit himself to look down, though the compulsion to do so never left him; nor did he allow himself actually to think. Mechanically--right foot, left foot, over and again--he shuffled along crabwise, watching the projecting wall ahead loom steadily closer.

As you can see in that quote above, Tom keeps his thoughts focused on his bodily movements rather than on thinking about what's below him or what could potentially happen. However, this strategy is really touch-and-go for him--he does look down, he does get incredibly frightened, and he does imagine what it would be like to fall and to die on the street. But even when these thoughts intrude on his consciousness, he keeps trying to dispel them:

It was extremely likely, he knew, that he would faint, slump down along the wall, his face scraping, and then drop backward, a limp weight, out into nothing. And to save his life he concentrated on holding on to consciousness, drawing deliberate deep breaths of cold air into his lungs, fighting to keep his senses aware.

Later, he keeps on trying to block out his mental processes and focus on moving his feet an his fingers:

Out of utter necessity, knowing that any of these thoughts might be reality in the very next seconds, he was slowly able to shut his mind against every thought but what he now began to do. With fear-soaked slowness, he slid his left foot an inch or two toward his own impossibly distant window. Then he slid the fingers of his shivering left hand a corresponding distance. For a moment he could not bring himself to lift his right foot from one ledge to the other; then he did it, and became aware of the harsh exhalation of air from his throat and realized that he was panting.

Tom's strategy of avoiding any real thinking doesn't seem to be a very good one, but since it eventually helps save him and get him back inside, we have to agree that it helped him avoid passing out and falling to his death, even when he stumbled.