You've gotten lots of great advice. I second the recommendations for Amy Daczyzn's and Jonni McCoy's books.
I'm a SAHM, and while I do have a very part-time job (church secretary; pays less than $235/mo.), we mostly just do our best to keep expenses low so that we can afford for me to stay home.
Some other suggestions would be to:
*Keep your thermostat a few degrees cooler (pile on those cozy sweaters!)
*Unplug unused appliances to avoid "ghost usage"
*Install low-flow shower heads and take shorter showers
*Shop grocery store loss-leaders and base your menu on them; stock up when things are on sale! Also, keep track of prices and be willing to go to more than one store to get the best deals.
*Rather than always buying something new, see if you already own something that will fill the need
*Utilize the library for cheap/free entertainment
*Save your change -- only spend bills
I hope you're able to find a way to stay home. I worked in a daycare for years, as well as watched kiddoes in my home, and I can tell you that nothing is more crucial to a child's well-being than being in the care of his or her own parents!

Good luck!!