If you can hold a plank for longer than 10 seconds, you've outgrown the exercise. Progress it with this resisted variation.
Holding the standard plank for ten seconds or longer is useless according to Dr. Stuart McGill. Unfortunately, many people still try to do planks for time.
A better way to progress this core exercise? Make it harder. The cable plank press will help you take your core strength to the next level, and you'll even build some extra upper-body muscle.
Total Core Training for Lifters
The World's Toughest Core Exercise