For upwind sailing-
Traditonal logic is that keeping the traveller high on a light air day with the mainsheet eased out a little will give the main an aerodynamic twist. This also gives the main a little belly to catch the wind. Never have the boom higher than the centerline of the boat, though. 'Loose is fast' and easing the backstay and cunningham helps, too.
In heavy wind lowering the traveller and bringing in the sheet as much as possible (but don't forget to ease it in puffs) will flatten the sail and spill wind out of it which de-powers the main. Tightening the backstay and cunningham help flatten the sail, too. Also, like PBzeer said, it'll help the helmsman with weather helm.
Downwind you want the traveller low and the backstay, etc. eased. The main is no longer being pulled in an aerodynamic lift put the wind is pushing the boat.