How do you unscrew really old tight screws?

Try spraying them with WD -40 first. Let it penetrate the threads for a couple minutes before trying to back them out. Most times this does the trick and is the simplest method.

If that doesn't work, try clamping a pair of vise - grip pliers on the screw head and back it out.

If that doesn't work, you might need an EZ - out screw extractor.

If you are able to heat the screw with a propane torch , that works sometimes but it's not always an option.

If all else fails, you have to drill dead center of the screw with a drill bit that is just smaller than the screw diameter and back the screw out with pliers.

Sometimes and this sounds counterintuitive. .. Tightening the screw and then trying to loosen it helps.

Using a screwdriver, I place a wide elastic on the thread head and it gives me slightly more torque.

A set of screwdriver bits for an electric drill with a reverse switch can be helpful!

I'm guessing that dynamite would also probably work.