Hidden loads from devices that aren't on can make up a rather large portion of your electrical costs. Doing this will reduce the amount of energy appliances consume when they're not on.

Entertainment centers and kitchen appliances are great places to target. Make sure you get anything with a clock. Anything that has a remote control or a "wall wart" (block that converts the power between the wall and the device) also should be on power strips.

Make sure to group items that are commonly used together. Like an entire media center, your toaster oven and coffee maker, or your home office. Be sure to not put things you use very rarely on the same power strip as those you use everyday, unless you put another power strip between them, so you can keep the rarely used device off when the other stuff is on, but turn everything off together (Thanks to Sam for this suggestion).

That is why you're doing this after all. Putting the devices on power strips doesn't do anything if you forget to turn them off all the time. Once you feel like you've gotten into the habit of actually using the power strips for at least a couple weeks, you're done.