Here are a couple of tips on how to correctly execute a "plank".
When you do a plank, focus on your breathing! First, make sure you don’t hold your breath. Second (and just as important!), make sure you’re not breathing “shallow” breaths.
By paying attention to your breath you will be able to hold the position longer – but you’ll also be strengthening those deep core muscle (i.e., your body working the way it is meant to … while breathing!).