Hi...Ther are a few ways of doing this.....One is by using the stray hum pick-up from your finger... You just amplify and rectify this to trigger what ever you need to.....The next method is the capacitive one, whereby you send a square wave signal into one input of a CMOS exclusive or gate (4030/4070), then the same signal through a 4.7Meg resistor to the other input (this input also being the touch plate)..... The idea being that when both inputs are high or low, there will be a logic 0 at the output.... When you touch the plate it will alter the time constant of the signal into the touch plate input, causing the output of the gate to produce pulses....The other method is by using a touch plate made in 2 sections and detecting the resitance change when the finger touches the plate...Hope this helps...Daniel.
There are several touch switch example circuits that can be found at http://www.discovercircuits.com . Click on the schematic link and search with the keywords touchswitch.