If you can setup your roof solar panels (be they solar thermal or solar PV) to follow the sun that should increase efficiency, but it might be marginal. I've also seen a lens system fitted over a solar panel that concentrates the solar radiation.
Next is to try and cut down on your power usage
Air-to-Air heat exchangers might be an option too, and certainly using heat exchangers to try and not flush your heat out might be worth a look.
EG when you have a shower the water that goes down the plughole is only a handful of degrees cooler than when it came out the shower head. Could you use a heat exchanger to take heat from the 'waste' water and pre-heat water going into your hot water cylinder?
EG If you run a clothes dryer, are you venting the air directly? Could you run that through a heat exchanger to pre-heat fresh air coming in?