The key to deception on the backhand serve is the elbow. If you are serving backspin, you should vigorously contact the ball on the downswing, as much under the ball as possible, with a motion similar to chopping wood with a backhand motion. But the swing doesn't end there – you should follow through sideways and up by vigorously pulling up with your elbow. This forces the opponent to have to figure out if you contacted the ball on the down swing, or on the sideways or upward swing.
Similarly, when serving sidespin or side-topspin, you should start with this vigorous downward swing (chopping wood), as if serving backspin, but miss the ball, and instead contact it on the sideways or upward swing with that vigorous upward pull with your elbow. This time the elbow pull gives you spin instead of just being a fake motion. Again, the opponent has to figure out where you actually contacted the ball.
This principle actually applies to most serves, where your racket goes through some sort of semi-circular motion (often very short and abrupt, so opponent can't pick up the contact), where often are more vigorous on the part of the swing where you don't contact the ball. It's just a bit more obvious with the backhand, where much of the deception comes from the elbow. (But you still need a vigorous wrist motion for spin.) One advantage of serving backhand – you are facing your opponent, and so can see what he's doing as you serve. Some players can even change their serve, or at least service direction, in response to the receiver changing position at the last second.