I started with a FaceTime plank timer. I called up Liz to ensure that I was keeping form (under a professional eye) while holding it as long as I could — before I exhausted my muscles and started to sag in any of the three key areas.
You don't necessarily need a trainer to watch you; you can record yourself with your phone's video feature, or have a friend, significant other, or roommate watch to make sure you're holding that solid form the whole time.