Do you know how a park swing works? What makes it go higher.
I haven't seen anyone that does. There's (shudder) physics involved.
To gain height, you're trading energy and momentum.
What you want to do, when you're at the bottom of the arc, is shorten the pendulum arm.
So you scrunch up, like everyone does at the far end of the arc.
When you do that, it takes considerable effort, because you're adding energy to the swing.
Then, at BOTH ends, lay out flat, to make the pendulum longer, so that at the bottom, the energy you added gives you more momentun.
It kind of feels funny at first, but you'll find that you gain height much faster.