A healthy battery can handle sitting for months without going dead, even with the tiny amount of current consume by the clock. So if your battery is going dead, you've got something else creating a draw in your car. I'm not really a big fan of battery shut off switches, unless mandated by the rulebook for a race car. I've seen the ones with the green knob fail both ways; open and closed. If you use the car with them a bit loose, they can create enough heat to melt the plastic insulator, then lose connection all together. If you repeatedly turn them on and off(like every day), especially if you tighten them too hard(like with pliers!), the insulator can get cut and they no longer disconnect. Not sure why I even posted the pic of one; I think they cause more problems than they solve.
If you have a rear mounted battery, a proper competition style shut off switch is the answer-
http://www.summitracing.com/int/parts/tay-1033