iirc, mostly people turn it on/off by connecting green (i think!) wire to black (GND). While its connected there is power. you could, in theory, keep it always shorted to GND, and simply use main switch. dunno if that's safe, but at least you'll save some electricity

in some rare cases you might need to use gray wire instead of black.
anyway, check that Ians page first, its excellent, very informative.
I've also used this in A1200:
http://aminet.net/package/docs/hard/AMI_ATXits more complicated (pic programming), but has nice extra features.
(don't forget that all switching PSUs need some load to work properly and all the usual stuff about electric shock hazard.)