A few options:
- install a continuous water heater before the tank.
- bump up the tank thermostat setting, if not already maximum. You'll dial the temp lower at the tap and use less tank water for a shower
- in addition to bumping the tank temperature, add a mixing valve or tempering valve at the tank to avoid scalding
- recover heat from the drain using "drain water heat recovery"
- insulate all hot water piping from tank to shower