What sort of boat Frank?
Sounds very much like our learning process with the Davidson 28 and our old MPS. All the above advice is good, especially not running too deep downwind, and easing out that sheet as far as you can without the forward edge collapsing. It is very easy to end up oversheeted after it flaps a bit and you correct.
We have also started adding a second tack line tied to the windward rail so we can haul it out from behind the main a bit, but that's a hack.
Ultimately we are now setting up a proper symmetric spinnaker as it can get so much more sail out downwind,