I'm not sure if this was written before. I've been searching on Dahon site and forums on what are the screws/nuts that should be tighten on a regular basis on a Dahon folding bike but I can't find one.
Hope this will help.
Unfold your bike. From the floor grab it in the middle fold then slightly lift it up and down. If you feel there's some movement in the middle then you should tighten the hinge screw. Fold the bike, there's a Phillips screw that will allow you to stiffen the locking hinge. Try to find the right amount of tightness. Making it too tight will not allow you to close the lever. Unfold it and check if there's still movement in the middle.
I hope Dahon used an allen head instead of Phillips. I feel that it's easy to mess the Phillips head since the bolt is a bit hard to turn. If you have a small spanner, i think it is better to tighten it on the other side.
Always spray WD-40, wipe it, then lubricate these hinges, bolts/nuts as they can easily corrode.