All I do is standard situps on a sit up bench. I do 4 sets, I don't count reps I just go until I can't do anymore because of the burn. IMO as long as you focus on getting a full stretch and tight contraction that is what matters. Work them like any other muscle. I personally don't like leg raises, its hard to keep good form towards the end and my hip flexors and low back start to burn more than my abs. There is really no need to "keep muscles guessing" if its working and giving you gains stick with it.