I'm not a big fan of Sit-ups and I can count on one hand the number of times I've done them in the last ten years or so. They are at best a cosmetic exercise and at it worse a great way to overtax your back. So what do I recommend you do to tighten up your abs? No surprise that I would recommend Kettlebells, the RKC Plank and the WOP (aka The Ab Wheel) right? Well - I'm not the only one who thinks that the combination of Kettlebells (in particular the Turkish Get-up) and the WOP are the key to a truly strong midsection. Check out this article by Bret Contreras on the T-Nation website for more information as to why these two exercises scored so high in their effectiveness in building a strong and resilient abdominal wall. BTW - surprising the Chin-up also scored very high in ab recruitment... you've been warned!
Here's an additional video explaining the RKC Plank if you're already familiar with the exercise: