Heating water is usually the second largest energy use in a home after heating the rooms. Because so much of it runs on habit, there is plenty to trim without anyone going without a hot shower.
Showers, not baths
A bath uses a lot of hot water; a reasonable shower uses far less. The exception is a long power shower, which can use as much as a bath, so the saving comes from keeping showers brisk rather than the format alone. Cutting a few minutes off a daily shower, across a household, adds up to a meaningful amount of water heated over a year.
Set the right temperature
A hot-water cylinder set far hotter than you ever use it just loses more heat standing there, and you end up mixing in cold at the tap anyway. Around sixty degrees is the usual recommendation, hot enough for safety against bacteria but not needlessly scalding. Combi boilers heat on demand, so the equivalent is not running the hot tap longer than you need.
Insulate the tank and pipes
If you have a hot-water cylinder, a proper insulating jacket is one of the cheapest savings going: it stops the heat you have paid for leaking away while the tank sits there. Lagging the exposed hot-water pipes nearby helps too, keeping the water hotter for longer between the tank and the tap.
Mind the immersion heater
An electric immersion heater is an expensive way to make hot water, because it heats with electricity rather than gas. If yours is a backup to a gas boiler, make sure it is not quietly left switched on, topping up the tank with pricey electricity when the boiler would do the job for less. Run the figures through the calculator and the immersion's appetite is obvious.