Find the geyser switch, and get into the habit of turning it off each morning or evening, just before your bath or shower. This will stop cold water flowing into the geyser when you turn on the hot tap. Turn it on again an hour or so before you need hot water again.
There are some great geyser timers available, which automatically turn your geyser on and off, in accordance with bath and showering times in your household.
Source: the Enviropaedia