I have a subfloor that is 7" T & G planks running diagonal to my joists. The joists are 24" OC. When I walk on that sub-floor, there is a lot of movement in the planks when I get to the middle of the joists. I went out and bought 1/2 inch CDX plywood, but am wondering if I should get 3/4 inch or some other type of plywood instead. Thanks for the help.

1/2" will be plenty to stiffen things up,

glue and screw..

I need to glue it? Seriously? I've never heard that I need to glue it...

Not many carpenters did this until recently. But using the right adhesive is a sure guarantee that the plywood wont loosen up over time. it's sort of like double tapping a zombie . Just want to make sure they stay dead so you put an extra bullet in the cranium.