This basic workout from Men's Health is a great way to build up your abs. You can do this in a corner or at the base of a weight machine.
Load a bar-bell lightly on one end. Wrap the other end in a towel and place it in the corner of a room or wedged under a bench. Once this is secure, stand facing the loaded end with the bar pointing towards you and hold the bar in front of your face. Keeping your arms straight, pivot your feet and torso as you bend at the knees to move the near end of the bar down and across your body towards the floor.
Reverse the motion to lift the bar back up to the starting position, then repeat on the opposite side. That's just one rep.
When lifting the bar to one side, the foot furthest away should pivot on the toes to help ensure a smooth and controlled movement. So if you are lifting the bar down towards your right foot, you should pivot on your left toe while keeping both knees bent. Inhale as you return to the standing position.