If you want to really dial up the effectiveness of your planks, then check out this video where I show you exactly how to do that.
There's two big things here that are more than likely different than how you have executed a plank before.
1) Hands open, fingers splayed on the floor
As opposed to making a fist and clenching for dear life, I like to have clients go with open hands and fingers splayed into the floor so that they can get the sensory feedback from the ground to properly push away from the floor. This creates a chain of events from the fingers, through the arms, into the rotator cuff and trunk that allow for better position and stability.
2) Exhaling as you go up and then continuing to exhale as you hold position
When a forceful exhale is used like in the video, the front side of the ribs "close" (internally rotate) which creates downward pressure into the abdomen and your ab muscles* (external obliques) along with your breathing muscles* (serratus anterior) "close" the ribs.
This will give you immediate feedback as to whether or not you're in the right position with the proper breathing to get into plank position and truly feel your abs tensioning up to maintain your position.
A Word of Advice: avoid crunching to get up into position. Yes, you feel a lot of abs, but it's compensatory. Press with the forearms, hands, and fingers (specifically the thumb and index finger) while focusing on getting the front side of your body away from the floor by pushing away from it and you will get all of the benefit that you need.
Exhale for 3-5 seconds as you go up, and then go back down to the floor before doing your next rep.
*These aren't the only ab and breathing muscles, but are major players in the context of this particular exercise and how it's executed.