Can you put anything in a 12 volt battery to help it hold charge?

No. Your best bet is to keep the battery clean. Wipe it down often. Dirt will conduct electricity from the battery terminals and slowly discharge it. If the battery is on your garage floor, make a simple wooden tray to put it on. Batteries find concrete a nice path to discharging. In a car, ensure that all lights are off when the car is turned off.

In the winter, use an extension cord and a utility light with a bare bulb and leave it turned on on the driveway or street underneath where the battery is. Warmth will greatly increase cranking power. In the old days when we had a naturally-occuring cold period of 25 years, I would crank the car engine for one revolution just to create a little heat in the battery. After about 20 seconds, I would start the car. This often made the difference between the car going and the car staying right where it was.