how do i get a tighter lower stomach?

if you already have a cardio routine keep doing that and maybe step it up a level and i hear that reverse crunches work pretty well.(those are the kind where you pull your knees into your chest)

I got a well defined stomach and abdominal muscles.. so i think i can help. here's what you do.. (assuming you already have a controlled diet and u do regular sit ups)

Have wheat bread instead of white bread..

Cut down on Rice.

try a NO OIL- NO SUGAR kind of regime for just 2 weeks..

trust me you'll see the difference soon.

Another thing- also try doing sit ups holding a weight in your hands..

Cheers!

Pull ups with a good diet and workout regimen.