Leg raises are a real staple lower ab exercises, and can be performed on either a captain's chair or a pull up bar. Position yourself in the chair or hang from the pull up bar, and raise your legs upwards, keeping them as straight as possible. Make sure you go beyond the 90 degree 'L' shape- the bottom part of the movement targets your hip flexors more than your lower abs. If you are struggling to perform the exercise, do not worry- it is tough. Instead opt for the knee raise- the same exercise except your legs are bent at the knee. Again focus on going well past 90 degrees- range of motion is key in performing this exercise.
I do not actually know the proper name for this exercise-my mate Scotty demonstrated it about 4 years ago and it has remained a staple of my abdominal workouts ever since. Take a 5kg plate, preferably one covered in rubber. Lay a gym mat out on the floor and lie face out, with your legs and arms extended. Hold the plate in your hands, draw both your arms and legs in, transfer the plate onto your shins, and then re-extend your body with the plate balancing on your legs. Now draw your legs back in and transfer the plate back into your arms- that is 1 rep- try 4 sets of 10-15 reps. If you find it tough, try a lighter weight, or indeed no weight at all.
The real benefit of this exercise is that it targets a huge range of muscles in your core- pulling your legs in works both your hip flexors and lower abdominals, and crunching up to grab the plate with your hands also works your upper core hard. Furthermore this is an exercise that you can make measurable progress- once you find 5kg easy, try and move up to 10.
Swiss Ball Roll Ins
Place a Swiss ball on the floor and balance your shins on top of it, keeping the rest of your body in the press up position- (as though you were about to form an elevated press up). Draw our knees in towards your stomach, allowing the ball to roll forward. Squeeze your abs and then re-extend your legs to the start position. Try 4 sets of 15-20 reps. This is a really effective exercise- not only does it work your lower abs , but your entire core has to work to keep yourself balanced upon the swiss ball.