Situps have pretty much been supplanted in my workouts by various kinds of crunches that all target their individual areas better: Reaching crunch, side crunch, bicycle, Russian twist, V crunch, etc. For my money the most effective all around single ab strength movement is the jackknife.

But not all ab exercise has to come from ab workouts specifically. The abs and glutes are big stabilizer muscles in the chain of movement that starts at you feet. Next time you're say, doing some standing dumbbell curls, flex your abs and butt to straighten your spine and correct your form. You'd be surprised how much extra burn you can get that way.

Also do lots of roundhouse kicks if you want that natural V

Source- was a small time kickboxer turned fitness trainer