
Another good exercise is when you lie on your back with your kness bent. then lift your bum off the ground until your shoulders to knees is in a straight line (diagonally upwards - only shoulders/head and feet should be on the ground). try crossing your arms over your chest as that stops you using them to stabalise you and makes you use your core instead.
Then, lift one leg off the ground so thats in a straight line with the rest of your body. The important thing is to keep your hips pushed upwards and to keep them level. The tendancy is to let one side drop down when you lift a leg up, make sure you don't! Hold the leg there for 10 secs then swap and do the otherside. Repeat as many times as you like!