I just started doing planks about 3 weeks ago..I can only hold my planks for 5-6 seconds each time, and I only do two planks a day, but I definitely think they would help flatten and tighten up your tummy, as it's certainly working for me. I've lost 5" around my tummy and another 5" off my waist since June 1st...with the majority of those inches really starting to come off since doing the planks.

If you look at my journal, ~ Onederland ~ I just started my own little 55 day 'plank' challenge TODAY ...your welcome to try it to see if it helps you.
My goal is to do two planks a day, holding each one for 6 seconds on the first day, and then every scheduled workout day (5 x weekly) increasing that hold by one SECOND....so hopefully after 55 days, I will be able to hold my planks for a full minute