As a bonus, getting good at planks will help a lot with push-ups (if those are something you're interested in working towards), because then you can already hold your body rigid and straight, so it's just your arms and shoulders that do the work.

Looks like I will get some more use out of my stopwatch in the future.

My theory is the best strategy for planks is to put your stopwatch somewhere where you can see it without strain, but it's not directly in front of you, and find something on YouTube to watch instead.

This somehow makes it far, far easier to hang in there than staring at the watch and grimly counting seconds. *g*