Hi,
The only way to decrease power consumption in LEDs without affecting the light output is to find LEDs that are more efficient. That is, they put out more light per amp, and/or they have a lower terminal voltage. Either way you get the same light output with less power consumed in the LED itself.
The power consumption of the LED is not the only issue however because sometimes the LEDs are driven from a resistive source. The resistive part of the source dissipates power too so you loose power just because of the way it is being driven. So the idea then is to reduce the resistive part of the driver. This is usually done with a switching voltage regulator set up as a current feedback regulator. This usually (but not always) provides higher efficiency in the overall system. I said usually because this isnt always the case. If the voltage source voltage level is close to the voltage of the LED or LED string, then there may be little or nothing to gain by using a switching regulator. In fact, there are times when using a switcher will actually reduce overall efficiency and this is when the source voltage is already pretty well matched to the LED voltage.
So the first thing to do is analyze the system to see what category your circuit falls under and take the appropriate action. It usually helps to find the most efficient LEDs first unless the lower forward voltage then provides a mismatch between source and LED characteristic voltage.
It almost always helps to run the LED at reduced current too. The efficiency goes up if you run it at 1/2 the nominal rated current, so running two LEDs at the same current as one LED effectively produces more light. The effect isnt super significant however so you may want to do a couple of tests with your actual LEDs to see if it warrants doubling the number of LEDs for a small gain in light output. Another benefit however is very significantly increasing the life of the LED.