Having trouble holding a plank I take my mind off the length of time I hold one by trying to do it for the duration of a record on the radio and by concentrating on my breathing. So far (this morning) I've managed 4;23 to Robbie William's Feel.
It works every time!
I've worked my way up to this time and remember to stretch my abs after.
This would work for a modified plank and side planks