My first wife and I (God rest her soul) used the basal thermometer method after our first child was born. It takes self-control, but it does work. We were able to time our next two children on our schedule. In a SHTF situation, I think it would be the ideal method. No "consumables" (condoms, pills, etc.), just a thermometer and a piece of graph paper, and in a really tight situation, you can even do without the graph paper. The graph paper just makes it easier to keep track of basal temperature.

The method has really improved since we started using it in 1959. Here are some links:
http://www.fertilitynetwork.com/ovulation/​ovulation-calendars.htm
http://www.babyhopes.com/articles/​basal-body-temperature-charting.html
http://www.babycenter.com/​chart-basal-body-temperature-and-cervical-mucus
http://www.fertilitynetwork.com/ovulation/​ovulation-calendars.htm

I would recommend AGAINST a digital thermometer in an SHTF scenario because they require batteries. Get a regular mercury or alcohol basal thermometer. No batteries needed. Take care of it so it doesn't break.

Good to see someone else using the method. But did you have to use the thermomter the whole time? I have never used it and am doing just fine. And your right about it taking self control! Very hard to do but worth it.

Yes, my wife took her temperature EVERY morning, and I dutifully entered it on the graph paper when I got home from work. We always knew where we were in her cycle.

If a woman's cycle is irregular, I'd be cautious about trying to take temperature just around ovulation. She might miss it. Better to take it every day.

I have three children now. My wife and I have talked about SHTF extensively and decided on our solution. Yesterday I had a vasectomy. We figured this was the best sure fire way.

That is a sure fire way right now, but what about for those that are just reaching adulthood and those who didnt have the surgery? They will still need to know how to prevent pregnancy. Also I'm not the kind of person who believes that we should give up on children just because the world sucks. The world always sucks. Life for the founders and the people of that time was brutal. And yet they changed the world by having an amazing idea. And if we refuse to have children its not going to get any better. We change the future through passing on our values, not by giving up.