If your snap buttons on an item of clothing open too easily, you may be able to make them tighter by using a small screwdriver, or better yet, something with a sharp metal point, to bend some of the tines slightly inwards. The tines make up a circle of metal, into which the side of the button with the protrusion is pushed into.
There are different kinds of snap buttons
Based on the ones listed on the above web page, the kind I'm referring to above is the "prong" type, which has a circle of metal tines. This kind is generally small and found on shirts.
There's another kind of snap button, the "S spring" type, where the protrusion side of the button is pushed into a hole which has 2 parallel wires on the sides to provide the tension (both of which are a single piece of thin wire, bent and positioned inside of the hole).
Pants and jackets generally have larger snap buttons of the "Ring spring" type, which use a single piece of thick wire shaped into a circle, to provide the tension. The circle of wire expands slightly outward against the containing round frame of the button, when the other side of the button is pushed in. That kind of button doesn't usually have a problem with becoming looser over time.