Move loose, fight closed.

I leave my cavalry in loose formation until I have the enemy unit I want to charge directly in front. Then I one-click for an attack. Then I tighten up the formation by hitting the "c" hotkey, because the loose formation helps get the knights pointed in exactly the right direction before they close up. This has the added advantage of avoiding some missile hits, which can cause knights to fall into the "Stop, we have to tidy our formation" problem.

Even after all that, the enemy starts riding away when they see a knight formation lining up -- a very reasonalble response.

That's fine with me. I just wanted to scare them back into my hail of arrows anyway.

If the target unit's pinned in melee, however, it dies.