Simply degreasing the screw holes and threads with lighter fluid will usually do the trick. If you want to go a step further, degrease and then put a small dab of Blue (No. 242) Loctite on each screw. The most powerful agent is aged tomcat urine. But obtaining it is a problem as aged tomcats often object to being squeezed.
Outdoor Skills – Hunting Q&A
I missed my one yearly shot at a deer because my scope-base screws were loose, and it's not going to happen again. Can you recommend something that will freeze them in place permanently?