I'm not saying this is going to make the halves fit perfectly firm together when closed, but it will reduce the wiggle and give on the right side of some Pre phones. In my phone's case it's the left side that was pretty loose.
You need a T5 screwdriver and a pin or small sharp object
Pull off the back battery cover
Use the pin to remove the Palm sticker on the screw on the loose side
Tighten with the T5 screwdriver
Replace the Palm sticker, be sure to orient it the same way
There is still a gap, but it doesn't shift when I touch in that area any more.