While Shifties are a snowboarding trick they are also an optical illusion. When doing a Shifty you are creating illusion of doing a 180-degree turn in air and reversing it before you land. Of course since it is an optical illusion you do not really turn 180-degrees.
Twisting your body in opposite directions once you are in air creates this optical illusion. While this can take a little bit to get used to it isn't as hard as it sounds. As with all aerial tricks key to this is timing.
The first step is to become airborne. This can be done by via an Ollie, a ramp or whatever else you happen across that will get you off ground. Ideally you should launch up in to air more than you want to move forward. Going up more than forward will improve look of your trick and provide you with more time to set it up and recover from it.
Once you have left ground you can begin to twist upper and lower portions of your body in opposite directions. The technique for twisting your lower body is much same as doing a 180. Push your forward foot to side that you wish it to spin while keeping your rear foot basically planted so as to act as a virtual pivot point. An important thing to bear in mind is to not twist your hips.
Twisting your upper body may be a little easier. The easiest way to twist is to simply point your head way you want your body to turn and rest of you will naturally follow. Once again be careful not to twist your hips, as they are anchor points for your body. If you do twist your hips you may find that you turn too far and thus have trouble giving impression of having turned 180-degrees or landing your trick smoothly.