Set up a barbell on the pins of a power rack (you can use boxes or rubber mats too, as shown here) so that it rests just above knee level.
4 Dumbbell Floor Sweeper
Sets: 3 Reps: 12-15, 8-12, 6
Lie faceup holding dumbbells above your chest. Keeping your legs straight, raise them up and out to the left. Repeat to the right side. Return your legs to the floor and do a situp, keeping your arms straight above you.