How do you keep them turned on? I've seen devices that you can connect to the ATX plug to turn them on/off but I'm curious if there is a DIY way.
If your power connector has 24 pins, jump pin #16 (it's always the one with the green wire) to any black wire (ground).
If your power connector has 20 pins, jump pin# 14 (again, it's always the green one) to any ground pin (black wire).
With the respective pins grounded, the power supply can be run stand alone, without a mother board and you can turn it on or off using the mechanical power switch on the back, near the power cord socket.
Most new powersupplies come with both connectors, so you could use the connector that's NOT plugged into your motherboard as a remote on/off swtich, by wiring a switch between the green wired pin and ground (one of the black wired pins).