I have installed a new 3/4" T&G plywood underlayment down over the existing 1/2"plywood subfoor which was resrewed. I am now ready for my new Hardwood Floors I hope? Is there anything more I can do to prevent future SQUEAKS and what is the best material to use between layers? I've seen felt and Rosin Paper used but there may be some new product I'm unaware of.

Just make sure the new 3/4" plywood is well nailed/screwed and solid. Over that, install #15 lb asphalt saturated roofing felt IF you are installing 3/4" T&G flooring. IF you are installing plank flooring of 5" or wider, consider gluing AND nailing. IF your floors are factory prefinished and you are using the #15 felt, butt the felt seams together instead of overlapping them 4". Nail the floors down with the factory recommended fasteners to the recommended nailing schedule.