How do I wire my light fixture?
Three way wiring diagram for 2 switches (courtesy @tester101)

In this diagram, power enters from the left.  If the switches are clock faces, the common terminal is at 7, the two travelers are at 11 and 1 and ground is at 5.  

Power flow:  Power enters left switch on black wire at 7, depending on switch setting, it leaves either on the black wire OR the red wire (This is the key to 3-ways: power can get to  the light by either switch, and can be turned off by either)

The power enters the right switch by either the red or the black.  It leaves the right switch on the black-tagged white wire at 7.  It then enters the light fixture(s) and returns to the neutral (white), back at the left switch location.

Fortunately, electrons move at the speed of light (nearly) and don't get dizzy. 

There is another scenario where the power enters at the light and and the hot (black, ungrounded) is 'sent' to the common of one switch and the white(neutral) is 'sent through the light and down to the common of the other switch.  Both travelers still must traverse between the two switches. 

In simplified form, two 'three-way' switches (SPDT) in schematic form (grounds omitted for clarity):