When my four-year-old gets overwhelmed, I invite him to put his hand on his chest and feel how fast his heart is beating. He's often surprised to feel it. Then, I tell him to take deep breaths (like he does when he's getting his bronchitis treatment by breathing into the babyhaler) and feel how his heart starts beating slower. He's usually impressed to feel this change in progress.
Another thing I noticed is that my son is finding it easier to calm down when I'm close to him, and keep a loving hand on him, as opposed to giving him space or walking away from him during a meltdown. He actually freaks out if I try to step aside and give him space. So, some of the things we can do are and must be child-centered, based on what we observe that works best with our particular children.