Also I'm a bit wary of wiring up mains - are there any precautions I need to take (other than obviously making sure I'm never touching ANYTHING when it's plugged in, not jamming my finger in the caps after it's been on)
From the plug to the power supply, use proper gauge wire (probably something like 12-14 AWG), and for extra safety, put in a fuse inline on the hot side (most power supplies have such a fuse already - check and see if it does, and if not, add it - use a standard fuse - not slo-blo - of an amperage equal to or a little less than the max current rating for the input side of the power supply).
I was thinking of trying to mount one of these:
Well - you would likely want the opposite version - based on the image at that link, that seems like a "female" recepticle, you would want the male (like that on a PC power supply).
in the panel. Then I suppose I somehow wire those to my power supply? Maybe I want a 12 amp relay that my switch drives which connects AC to the power supply?
You are supposed to use a DPST switch, with hot and neutral connected to each half of the switch, so that both are switched at the same time (do not put the switch on only one or the other side; that can create a dangerous situation in certain failure modes); look at how old AT-case PC power supplies switches were wired.
Just want to be as safe as possible... What wires to use, what insulation to use, what is a grounded plug? (Three prong?)
Three prong, yes - is a grounded plug. If you are confused by any of this, or have other questions, it would be best for you to do a lot more research, and to also see if you can find somebody local to you to show you how things connect up. Mains power isn't something to be casual about; it isn't difficult to work with if you know what you are doing, but if you don't, or if you are careless, it can bite you, and hard. You can get everything from a hard tingle to death if you're not careful and understand what you are doing. There is a lot of information out on the internet on how to do all of the above. The basics, though, are (bad ASCII art ahead!):
| \ / / +--------------+
==| |-- Hot --------+/ / +-------[FUSE]-----| |
| Plug |-- Neutral -----+/ +------------------| Power Supply |
==| |-- Ground ------------------------------| |
Hopefully this will illustrate what I was saying, but it would be best to study this and other information online (as well as at your local "big box" home improvement center on electrical wiring), before trying anything (for one, you need to learn how to identify the hot side and the neutral side of the plug).
Good luck, and if you are at all jittery about something, stop, and take a look at what you are doing before you plug anything in.
NOTE - In my illustration above, the PLUG is supposed to be a 3-pronged plug, not 2 as was illustrated for brevity...[/edit]