The background illumination and the radio chips are the largest energy consumers. Mainly because they are on when you use the device.
Grayscale would improve battery a little if and only if you would constantly pull down the notification center or do similar things that use composition of multiple transparent layers.
If you turn on Airplane mode, your battery live will improve dramatically, because you don't switch cell towers or look for WiFi signals.
Whenever I drive or don't use my phone I'll try to do that.
I also try to be a little less available to people. They have to learn to call me back or not expect a response within 10 seconds.
But that's related to age, I suppose.