Get the First Out
This may seem like painfully obvious advice, to the point of being useless -- get the first batter of each inning out. It's like saying, "The key to pitching well is to pitch well." The key here is the emphasis on the first out. Obviously, you want to keep every batter from getting on base, but that first one? That's the important one. That sets up your whole inning.
If you get the first out of an inning, it accomplishes several things. It allows you to work on the second out without having to worry about a runner stealing a base. It also allows the defense to play better defensive positions, not having to worry about a runner going from first to second [source: Schlossberg].
If the first batter of the inning does get on base, the offense has more options. Sacrifice plays can be used to advance the runner. The offense is less vulnerable to a double-play (they can still happen, of course, but it won't end the inning). The threat of a home run becomes much larger. So when that leadoff batter comes up to the plate, it's really time to focus. Throw strikes and make sure you don't walk him.