Posing is an extremely important aspect of a bodybuilding competition that's too-often overlooked, particularly by those entering a bodybuilding contest for the first time. All of the careful preparations you've completed prior to a bodybuilding competition (strength training, cardio, dieting, maximizing muscle definition, mass, strength and density) will be wasted, at least in part, if you don't understand how to properly hit the various poses.
As a rule of thumb, you should practice your posing at least once each day for 10 weeks prior to the contest. Doing so will not only help you to learn the poses and transition between them naturally, but also increase your motivation levels and remind you of why you're putting in all this hard work in the first place.
Remember, posing in a competition is much more difficult than posing at home. During the contest, you won't have a mirror, you'll be blinded by the stage lights, the temperature could exceed 100 degrees, and the judges may request that you hold each pose for up to 20 seconds. Without the proper level of practice and conditioning, you'll end up shaking and cramping while you hit your poses. This will be unimpressive to the judges, but it's something you can fix easily simply by practicing ahead of time.