Load your plank for better results
"MORE ISN'T BETTER - ONLY BETTER IS BETTER"
Planks are a great exercise to teach isometric tension, and improve core strength. Most people however, default to just trying to add TIME - holding their plank for longer and longer as a means of progression. Now there's nothing wrong with holding a plank for more time, but it doesn't automatically mean the plank being performed is structurally sound.
I am an advocate of loaded (weighted) planks, because they test the structural integrity of your plank, and instantly necessitate the correct tension you're seeking. Think about it: if you can hold a plank for 10 minutes, what are the first few minutes doing for you? Nothing really, other than serving as a route to a point where the exercise starts to become challenging, and thus promoting change. Why not find a way to skip that step, and go straight to the challenging bit? Be a bit more efficient with your time.
Loading a plank also teaches you how to plank correctly. Like in engineering, a bad structure will collapse under load, a bad plank will do the same. This also means loading a plank can turn it into a self-correcting exercise, as you will figure out how to position yourself to withstand the load and maintain position.
Of course you can still add time, once you have a good structure and understand what it is you're trying to achieve - even if your training partner gets bored at being used as the additional load!
Remember, ONLY BETTER IS BETTER, now go and enjoy your training!