How should you challenge yourself more on plakning exercises?? You can do two things. Increase the time or increase the difficulty of the exercise. I prefer the latter (once someone can hold a plank for 30 seconds). Plus who wants to do a plank for 1 minute +??
There are endless ways to make planks harder but I do think people can get a little carried away with this. I mean, how hard does it need to be? We program planking exercises in between strength exercises as a rest/recovery exercise. So I have no problem with them not being the hardest exercise in the world.
Here are a few ways to make planks and side planks harder: