Planks SUCK. They do. But they are a mental game. You can build planks up relatively fast with consistency.
My biggest advice for planks: When you do planks and you feel yourself sagging, tense up for ten more seconds, let it burn, and then STOP. STOP. Then rest before going again. There is absolutely no sense in propping yourself up on your elbows with your back sagging just until the timer runs out. You aren't getting stronger, and this will not work on the test. Best to do TRUE and HONEST planks for less time.
I also like doing side planks, left and right side. I don't hold these as long, but they help. The ever-so-sligth adjustments you can make one inch to the each side stealthily helps.
Here is a video on the correct form. It is an official Marine Corps video.
You can have your palms on the ground or in fists, just not clamped together. There's a Marine Corps Times article with Marines doing planks with their hands together. This is WRONG.
The max time used to be 4:20 (lol). The video says 4:20 but this was changed to 3:45 for the max time.
Actually you know what? I changed my mind. I'm not posting the plank chart fuck that. Everyone maxed the crunches, so you should max the plank. 75% of Marines in 2021 maxed the plank. This category is the easiest to max. I guarantee with consistent practice over three months, you will max it and get easy points for the PFT. Unless you do push-ups before. Do pull-ups!!!