I remember the frustration of not being able to flat-base when I was starting out. Loose & balanced is the name of the game. You need quick reaction time and that just comes with practice & developing good muscle memory. Leo's preferred lesson will not help you unless the terrain is also really smooth you need to be able to react very quickly to bumps, ruts, etc., that will want to redirect you and throw you off that balance resulting in the dreaded edge-catch-to-scorpion.
Also remember you can pressure an edge without being on an edge.