Let me say, of you have a lot of hardware, and you don't have a modular power supply, it isn't that easy. You can pretty much bend or twist any cable how you want, but you don't want to have to twist it more than once.
Another consideration is replacing or reconnecting hardware. If you tie everything down, and have to replace a piece of hardware, you'll likely have to do the job over.
Most important with today's systems is airflow. You must keep air moving through the case. In through the bottom (front) and out through the top (usually in the back, but better if you have a blowhole too).
I like to use unused power supply cables to hold all the other wires in place. It's nice to have a power supply that has the wires wrapped in shrink or mesh. Rounded IDE cables make everything nicer, but you can carefully fold ribbon cables (once!)
Tucking the small wires under the mainboard helps too, but don't wedge anything.
You can tape velcro onto the most unruly wires, and put the other end of the velcro on the chassis. Cleverly done, this offers some of the neatest results.
Hope this helps, and I'm sorry I repeated some of the other ideas already posted. Here's mine, flash off!