Focusing on core strength is really the focus of this month because I like people to understand that you need a strong core for great abs. But you should not focus on just crunches because they mostly only work one group of abs muscles (the front).
The bridge is one of those exercises that is amazing for not only the abs, but the bum, and the obliques as well. It forces you to balance and that's what develops that core strength. You can either choose to hold the position for as long as you can (target a minute or more), or turn it into a motion exercise.
If you choose to hold the position, start with your bum on the floor, and heels on the floor. Lift your bum off as high as you can, tighten your abs, and plant your heels in the ground. Hold the position. You may choose to make it harder by lifting one foot off of the floor.
If you choose to turn it into a motion exercise, do that repetitive motion of lifting your bum off and dropping it down, tightening your abs and bum each time you lift. When you lower down though, try to not let your bum go all the way to the floor. See the video below for a demonstration.
Cheers Eights & Weights!