Pete also knew when to step up return games, if he wanted to crush an opponent, he would go after them every game, if he wanted to conserve energy, he would focus on certain games to raise his returning level. He was a smart player.
An example of his returning game.
I want to add something to this. I think there are two aspects to the Sampras game that were more important to his success than anything else:
1) His amazing hold game, which means not just his serve, but his ability to hold serve. To never lose focus on his own serve, and one of the key reasons that he was able to do this is in the next point.
2) His attitude towards return games. Basically Sampras was so sure in his hold game, that he felt like he could afford to take mental breathers in the return games. I felt that his tactic was pretty much to focus his energy and take some fairly big risks on the first couple of return points. If things didn't go his way he would relax and more or less let the opponent hold. If things did go his way, he'd focus on trying to get the break. This especially happened early in sets. Later in sets he would focus more on breaking.