What is the best way to get a new glass screen back on tight.

Are you making sure the metal plate is getting screwed down tight and the digitizer cable is folded properly at the top? Those are a couple of thing that may help.

I've never had issues getting it to stick well. Always put in new adhesive and I recommend using a head gun to warm the new glass (and therefore adhesive) and then put some pressure on the screen for a while to get the heated adhesive to take well. I generally put the iPod face down on a towel that is lying on a flat surface. Then I put a couple of heavy books on top of the iPod. Always works.