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.
Eric describes how doing the YMCA song can help shoulder stability, why "your balls still suck," the right way to do a push-up, and how to do a DB Overhead Reverse Lunge.