Feigning is an excellent technique, and when used sparingly will almost always catch your opponent off guard. The great thing about feigning is that it can be applied to almost everything in this Wikibook so far.
A basic, but very effective, feign is the placement of your service. Very few players will look at your paddle to determine where the shot will go. Absolutely critical to making the feign believable is your eyes. You must look at where you want your opponent to think the shot is going to go. Also important, but usually only caught by players who have had feigns used on them for a while, is not interrupting your service routine. Don't change anything, and your opponent won't know when to expect a feign.
Rally feigning is a bit different from service feigning. It is more versatile, however. A common feign is used when performing the runaround. After settling into the forehand-backhand pattern, interrupt it by hitting to the same side (instead of the opposite side) that your opponent returned from. This works flawlessly on unanticipating opponents.
Feigning spin is more complicated than feigning placement. First, your opponent must be advanced enough to recognize spin. If you can convince your opponent that you always spin, you can win critical points by not spinning the shot. They will anticipate the spin and either completely miss the ball or miss the sweet spot.