u may wanna check out the other ab topics for more detail, but basicly...
its not a matter of getting "BIG" abs. its a matter for seeing them...
so cut down ur calorific intake, work ur core muscles 3times a week and do some cardio work. you need to get rid of that layer of fat!!!
its important to work your lower back the same/or simular amount as ur abs and obliques. otherwise u'll look silly and may cause spine problems due to ur front muscles being more tense.