Hey, who knows how to make clear coats to make paint last longer?
If you mean make the paint in the bottles last longer, use thinner instead. Usually cheaper (non-professional) branded model/acrylic paints need to be thinned when used well anyway. I usually don't bother, but that's because I'm lazy.
If you mean clear coats over painted figures to protect the paint, then it depends on how you painted. I paint with Testors Model Master Acrylics by hand with brushes and then finish with Testors Semi-Gloss or Matte Clear Acrylic. This is brushed on after the paint dries just like another layer of paint. You have to be careful not to apply too much because it can act as thinner itself and remove your paint or it may get too thick and start to look like you painted a thin amount of white on your model.
You can also buy spray cans of clear acrylic sealer if you are trying to protect larger pieces like plates. I wouldn't use it on figures though. It doesn't get into enough of the cracks, and it can leave your figures feeling tacky.
If your concern is on protecting inner joints, that is a whole nother ball game. You want to take the joints apart, sand down the connective parts, paint, and then apply a thin coat of protectant. Some people swear by super glue. Some by clear nail polish. I find both of those cause melting and can have issues. I try and avoid painting joints that will have a lot of rub as they tend to wear down normal paint and clear coats easily. And with the harder plastics, used as part of the Mega Construx joint system, paint tends not to want to stick to those parts at all. Instead I try to find a closely matching part and swap it from another figure. You could also just try painting joints with colored nail polish; just be careful of melting.
Bonus Painting tip: Don't use Sharpies on Mega figures. It never dries to the heads and becomes a smudgy mess. If used on the bodies it is permanent but bleeds over time.