old school but it works... work on your breathing

if you breathe correctly when you throw any punch you should know to exhale... it helps to tense the abs and also expels as much air as possible so if you do get a gut shot, your not going to be winded....
This is why you hear boxers sounding like an old steam train, releasing air with every punch

train your abs and obliques, a hard midsection is a solid midsection, a guy with a flabby midriff is going to soak up every shot... i've heard of a guy with a midsection so hard that it hurt the other boxers hand punching it.

When doing ab work, get a 'friend' to bounce a medicine ball off your gut. It will teach you to tense and expel air correctly and after a few times of being winded (when your gassing and form gets sloppy) it starts to resemble a real scenario.

With all muscle workouts, equality and balance is the key, train your lower back (erector muscles) as much as you train your abs. Your lower back has to be in balance as a strong abdominal wall will pull your torso down and can lead to posture/back problems after a period of time.
By doing exercises as hyperextensions, deadlifts you can balance out and create an ever harder midsection

Exercises for the lower back
http://www.exrx.net/Lists/ExList/Wai...#anchor1945210

Hope this helps