I would stay with the weighted stuff, but watch your form or which exercise you choose. Definately put leg raises in there. Preferably hanging leg raises. Situps or crunches.
Personally, I wouldn't do the 500 situps thing. Use weight like you've been doing.
When doing the reps, I keep my midsection tight and concentrate on the muscles contracting instead of letting my gut stick out and just doing the rep.
I think it was Needsize who does very little for his abs and has a great midsection. He did/does.. decline situps with a lot of weight and does a negative or very slow rep... I forget, but not too much ab work but with heavy weights.
People will tell you it's the diet. gl