MOTM

Okay,  I'm going to make my suggestion of how to do this relatively 
safely (Note the use of the word relatively).  This is still not as 
safe as not doing it at all, and I'd like to point out this is not 
how I did it on my synth.  I have a completely external rack mounted 
supply which has its own power switch on the front (But I did build 
it and I did wire my own mains.)  
The reason I am going to tell you how to do this is because I fear 
someone is going to try it anyway and get hurt, so if your going to 
muck about with mains wiring you might as well do it right.
Get a switch with large lugs with large holes in each lug.
Assuming one wire of the mains is hot, one is neutral and one ground 
(as is the case here in the states) run the hot wire to one lug of 
the switch, stick the stripped wire through the hole in the switch 
lug and twist it back on itself.  Solder the wire to the lug and to 
itself.  Now put a length of heat shrink tubing over the lug and the 
wire so that the entire lug and the entire exposed portion of wire 
is covered.  Repeat with a second piece of heat shrink again make 
sure the entire exposed portion of switch and wire is covered.  You 
might even repeat this a third time with the heat shrink.  The idea 
is to make sure the thickness of the heat shrink exceeds the 
thickness of the original insulation of the wire.
You want to make sure there is no exposed metal which could be 
touched under any circumstances.
Also be very careful that the wire (if it is stranded) remains 
twisted together prior to soldering so that the is not a loose 
strand which might either penetrate the heat shrink or remain 
exposed.
Repeat for the second lug with the return wire.
Now get a large piece of heat shrink big enough to partially cover 
the body of the switch itself.  Run the large heat shrink tubing 
over both wires and the body of the switch and shrink.
Test with an ohmeter to make sure the switch operates properly and 
the wires are not shorted together.
Obviously some types of switches will require mounting before the 
wires are attached.
Be sure and use a switch which has a high enough voltage and current 
rating for your application.
I use led type indicators on my plus and minus DC supplies, I think 
this is more useful than an indicator on the mains.
Hope this helps.
Please be careful.
I feel obliged to add a legal disclaimer.
You do this at your own risk.
I'm a degreed engineer and have worked for a utility company as have 
several of the members of this list.  I have worked with voltages 
close to a million volts.  I still try to be very careful when 
messing about with 110 or 220.  
One other note please be sure your power cord is unplugged before 
trying this.
Paul Haneberg