3M 77 Adhesive...
Spray it on, let it sit for a few minutes then come back and apply your sandpaper. I use it all the time at work to stick sandpaper to whatever I need to sand the places I need to get to. Aluminum rod, aluminum plate, etc...The stuff will stick sand paper to anything...even glass.
If you have a hard time removing the old sand paper, 3M Citrus Adhesive Remover is great too. I have both cans will reach of me at all times at work.