According to Nate Helperin, of TheStreet.com, a postnuptial agreement might save your marriage. For his theories and advice, see A Postnuptial Agreement Can Save your Marriage.
Do not, however, regard his advice as the last word in your case. His comments are generalized and not state-specific. He suggests, for example, that a postnup cannot address a waiver of alimony or spousal support. While that may be true in some states, it is certainly not true in Michigan.
For an earlier comprehensive discussion, see Will a postnup be enforced it it's not fair? posted on this Blog on December 13, 2007 and the series of earlier articles linked there.