One silly detail that I added myself as it was not in any checklist I've ever been presented with: check seat adjustment. Forgetting this brought me some, err, less comfortable flights, and once one is in the air it is too late. This is of course relevant in planes flown by several pilots. I made it the last point of the walk-around, or the first point of the cockpit-checklist.
Almost as important is to have the seat adjusted for any pax.