The Plank
For me, the Plank is both the easiest and hardest thing to do. It is easy to do because you don't need any equipment, and the hardest because it destroys your abs, and strengthens your entire body - it makes the core pop, strengthens your lower back and builds up your shoulders.
Once perfecting the plank, you can amp up the intensity by widening your stance and bracing yourself with your hands instead of your forearms and elbows.
To Begin:
Bend your elbows 90° and rest your weight on your forearms. Your elbows should be directly beneath your shoulders, and your body should form a straight line from your head to your feet. Think as though you are being shot out of a cannon. Hold the position for as long as you can. Your goal should be to hold it for two minutes.
The plank helps develop strength in the core, shoulders, arms, and glutes making it a great prerequisite for lifting heavy weights or playing intense sports. Even though you aren't moving or lifting weight, you have to constantly squeeze your abs to hold the position—most people can't last 30 seconds on their first attempt.
To Improve:
The longer you can hold the plank, the more resilient your lower back will be to injury, and the better your abs will look once you burn the fat off them. Follow these tips for longer plank times.
Hold It
If you don't have the core strength yet to do a regular plank, you can build up to it by doing a bent-knee plank. If you can hold a plank for more than two minutes with ease, you can move on to these tougher variations.
Lift one leg up. By simply raising one leg in the air, you dramatically increase the demand on your core to fight your body's natural urge to rotate.
Lift one arm up. Again, your body will want to fall to one side. Don't let it.
Reverse Walk. Walk-in reverse by placing your hand a 1/2 a palm behind the other while moving your opposite foot the same distance (see video)