Most T Nation coaches agree that the standard plank doesn't do much for you once you can hold it for 30 or 40 seconds. But here's a way to ramp it up.
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. So try this:
Dumbbell Pull-Across Plank
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.