How can I make a mixer shower more powerful?
The water supply comes into your home with a fixed pressure from the utility and then is diverted to your hot water heater and cold supply.  Assuming everything is in working order and properly, without a tank/pump, you can never exceed the utilities pressure. 

If your cold water supply puts off a lot more pressure than your hot water, this likely indiciates another problem like a partially closed valve on your hot water tank, a blockage or inproperly sized pipes. Your shower mixer (assuming 1 handle) balances the pressure so that you won't get scalded if someone flushes the toilet which means it is unlikely you will be able to get different pressures out of the valve when mixing the two supplies.

I would suggest that you verify that all of the valves for your hot water tank are fully open.  You might also try turning up the heat of your tank so that less hot water is required, but be careful when you do this as you can make the water dangerously hot.

Your shower valve might have some adjustments on it too.