I had a loose terminal and found a simple solution.
Open the battery cable clamp and clean between the plates that come together as you tighten the clamp bolt. I had some corrosion in there. Then push down on the terminal clamp as you tighten it back on. Mine is now very tight once again.

Just a few extra notes, I also noticed the two "plates" had a shape to them. You need to make sure they come together correctly. Also always make sure to open the clamp a modest ways (say 1/8 inch) so that it is initially very loose when you put it back on. Its important to push down on the clamp as you tighten it because Top post terminal batteries have tapered posts.