Don't get ahead of yourself. Walk up to the plate with confidence. I'm ready for this. If the player jumps in without getting a sign from the third base coach, he's getting ahead of himself. If he's tense or distracted, it shows through his body language.
Good coaches know when their players are following a routine or breaking from it.
Then make a commitment to the sign. If it's a bunt, visualize it. After you get your sign, smooth the dirt in the batter's box to wipe the slate clean. Take a deep breath, exhale and step in. It's a new at-bat, and you've got a mission.