Other than actually activating your core (and shooting for intensity over duration), the key to making planks not suck is to keep your mind busy, whether that's lifting limbs, walking around, or moving weights.
This exercise will hammer the trunk when you maintain a rigid plank. And once you've got that down, you can raise the difficulty by increasing the load or moving the weight in front of the hands.
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You see dumb guys doing this every week in the gym. Don't be one of them.
Combine rope pushdowns, overhead extensions, and dips into one giant set for guaranteed gains. Do 4 sets, 12 reps for each exercise in the giant set.