Push Like You're Walking
To keep the board from sliding out from under your feet, keep your movements simple. You can push to build speed just by taking your front foot and making a basic step forward, as though you were walking. Instead of pushing the skateboard away from yourself and losing control, think of your motion as sort of like being a gondolier pushing a boat along, and work with the direction you're already going. Maintain control by making fewer big, powerful pushes instead of a flurry of lots of little paddles.