Jeff has it pretty spot on.
I'd also like to add a few comments.
When sailing in flat water and steady wind the mainsheet stays where it is pretty much.
If it's super puffy you can play it a few inches or leave it out a few inches.
Whatever you do, don't constantly play your traveler, it's slow in all conditions. The only time you should be dumping your traveler is to de-power (again in puffs or if you're trying to sail really high).
As you open up your leech you drive your bows down. This is okay in flat water. However, in wavy conditions, where you are really pumping the main downwind, sheet harder when coming up to a wave or you will push your bows into the wave and slow you down a lot more. At the top of the wave you can ease a bit to open up your leech and push your bows down. But this is not as important as keeping it tight to get up the wave in the beggining so if you can't stay on top of it just leave it tight.
You mentioned you sheet in tight as if you were going upwind, it should never be as tight as upwind, just a bit off of that. Your always going to ease off just a bit. As it gets windier you are going to ease off less and less from your upwind trim. In 20+ you're really only easing a few inches.