Memorize the formulas.
Sure, practice deriving them too when it's within grasp. (It is not always within grasp -- derivations often sit much higher in the skill tree.)
But even if you can derive the formula, that doesn't excuse you from committing the formula itself to memory.
The greatest educational life hack is learning high school math in middle school and university math in high school. But even just being a year or two ahead has shockingly high value. If you know, you know.