I've used shelves for them in the past.
Using sticky velcro makes it easy to move things as you need.
I've also used lacing bars to cable tie (or velcro strap) them to.
Sometimes it just gets a bit messy.
I try to buy equipment that has integrated PSUs (so everything is on IEC).
For everything else, I try to find an inline POE adapter (you get them to USB, as well as a few standard voltages and barrel-dc plugs). Has the benefit of not using up power sockets, and lets you power cycle it from your network switch.