This guy has a few good ideas:
You could shut off the wireless transmitter (if it has one and you don't use it all of the time).
He also reduces the supplied voltage to his modem with a bit of a hack, but it is more than I would wish to do. If you were going to go that far, you may get some benefit from one power supply that provides DC to all devices.
Some ideas from me:
If it is available for your router, you can install
dd-wrt which allows you to do all sorts of nifty things with it. It has some settings that would affect power consumption, mostly to do with the wireless transmitter.
You can check if your device is supported here.
Also, 24 watts seems like an ideal application for solar - you may want to check out
Xringer's series on his backup solar project.