I'm not speaking from the voice of experience, but a friend swears lavender grows best in the concrete blocks she lines her veggie beds in. Since the bottoms are open it insures good drainage. In a pot I'd make big holes with a electric drill attachment like this, and mix extra pearlite with the soil.

Anybody else have some tips for growing lavender?

IThe roots absolutely do not like sitting in water. We used to plant the crown of the plant over a rock and drape the roots down over the rock. As mentioned above, the pot must drain really well.