Liquid Lust'er is an air cured coating material. Contact with oxygen will begin the curing process and the chemical reaction will use up any available oxygen it can to proceed with the curing process.
In that sense, every time you open the can of Liquid Luster, you are starting to cure the material. Even when you close the can, the material will use any of the trapped oxygen within the container to continue to cure (this is why you oftentimes have a hard layer of material covering an uncured layer below).
There are some steps to take to decrease this from happening. Make sure to pour out of the main container only the amounts you need to use at the time and do not leave the main containers open to the air.
When you use material out of main container, you can use glass marbles to fill in the excess space created in the container to prevent trapping of too much air into the main container.
We've also heard of customers having good success using canned air sprayers. You spray a burst of the canned air into the main container before capping it off. The canned air will evacuate the oxygen from the container and prevent the material from curing inside the container.
These steps should help prolong the life of the materials you've bought.