What I mean by the prenup not applying during marriage is that she still gets half of the wealth accumulated during the marriage, regardless of what the prenup says. The prenup only protects the wealth you already have going into the marriage. Most newlyweds are broke, which is why most people don't need a prenup.
There is one exception to that rule. If you own a business before you meet her, you may exclude the wealth generated from that business during the marriage from being included in the marital property, if the woman never has anything at all to do with the business. She can't ever have any direct access to the business accounts or participate in management in any way.
Vitor's three points are all correct. One that I would add is that the prenup must not be one-sided, or else a judge will throw it out. If you want a prenup to stick, write in something like you have to pay her alimony for six months after a divorce, no exceptions. Make it a token amount, maybe $100-$200/month. A clause like that can make a prenup hold together in court, even if you are paying $1,000 in alimony, which is tax-deductible in the US, to keep her from getting half of a $100,000 business.
The most important thing I could tell anyone about divorce is that there is no winning. There are only degrees of losing. The friendlier the divorce, the less it will cost you.
