Sleep. I'm naturally nocturnal which doesn't bode well in a diurnal world, so I take melatonin.
Otherwise, a reasonably healthy diet with a balance of carbs, plant and dairy protein, and some fruits and vegetables. On a longer ride (3+ hours, or 2+ hours at tempo), I'll throw in some sugary type power or candy and electrolytes.
I've learned in the past few years that some days, I simply have no energy. On those days, it's best just not to ride, unless there is some event that I'm committed to.