Here's my advise:
punch your opponent when he's coming in,
and cover up more.


Look man.. You only get the 'breath knocked out of you' when your hit in the stomach bag, which is just below the chest. You don't get the breath knocked out of you when your hit in the stomach muscles, which are just above the waist.
So, sit-ups and shit will keep the stomach muscles firm, and this is good. But if you want to prevent yourself being winded, then you need to look at your own boxing both offensively and defensively. When people work the body in boxing they have to sacrifice taking some head shots too, so you shouldn't let anybody have free range body shots on you - hit them in the head man, HARD. Also, a good guard should protect not just the head with your gloves but also the upper stomach bag area with your elbows - or be able to hover downwards naturally when required.

More to consider.. If you are too upright it makes it easier for your opponent to hit you in the stomach bag area and wind you, whereas if you crouch more it leaves your body that bit more protected.