Kick within your slipstream. Wider kicks increase drag. It used to be said that large, slow kicks were more efficient than short, fast ones, but this is not necessarily true. Keep your fins within your slipstream and, if you have to go faster, increase the frequency of your kicks, not their size. Use efficient fins.
Some fins deliver more thrust for a given effort than others.
See past ScubaLab tests. Both the bottom and the surface cause turbulence that robs energy if you swim too near them. If you need to make a long surface swim, do it on your back so your fins will stay submerged for their full strokes. Manage currents wisely. Learn how to detect, avoid and cope efficiently with adverse currents.