You can change the physical look of your 360 by purchasing new faceplates that snap onto the front of the system. There are a number of faceplates available, but you can also custom paint one yourself which truly makes it your own. Faceplates will run you around $20 or less. You can also buy special skins (fancy stickers) from sites like Decal Girl.
You can also customize the look of the Xbox 360 dashboard software by changing the colors of the guide backgrounds, changing the look of the content blades, and you can even use your own pictures as backgrounds. You can import pictures in a few different ways - attaching a digital camera to the 360 via the USB ports, burning photos to a CD, connecting to a Media Center PC, or using any kind of other USB storage device. My personal favorite method is to use a Sony PSP. The backgrounds for the PSP are already in widescreen format and look pretty decent even when blown up to TV size.
For detailed info on changing dashboard themes visit Xbox.com.
