Natural family planning can be very sucessful. It will take her a few months to really get her temps down to where she can see a pattern. But after she has done this, if you use this at the same time as condoms you should have no problem.

You should probably keep a dose of plan b on hand just in case. You don't want to be scrambling for it at the last moment.

Natural family planning can be effective, but it often isn't. Alone, its failure rate is 1% to 25%. http://www.drugdigest.org/DD/HC/Additional Content/0,4054,678,00.html 25% is a pretty high failure rate for birth control. It depends on how you do it though. A lot of people try natural family planning, but they don't know much about it, or they do know but they get careless. Natural family planning is most effective when you talk to a professional to learn more about it and when you make sure you do what they tell you and don't slack off.

More info on that: http://familydoctor.org/126.xml

Condoms are also effective on their own when used as intended, about 97%. So using both would be a great idea. Since the natural family planning method tends to give people a lot of trouble, you two could use a third method of birth control as well if want, or just until you get the hang of it. There are many other methods of non-hormonal birth control than just condoms and natural family planning.

Check out all of these options:

"Cervical Cap (a small, cup-shaped latex device that is fitted into the entrance of the vagina to block the passage of sperm)

Diaphragm (a flexible, rubber device that is held against the opening of the uterus by a spring to prevent sperm from getting into the uterus)

Spermicide (vaginally-inserted chemicals that inactivate sperm - often used in combination with other forms of contraception such as cervical caps, condoms, diaphragms, sponges, and withdrawal)

Sponge (a thick, soft cushion of polyurethane foam that is inserted into the vagina to block the passage of sperm into the uterus)..."
http://www.drugdigest.org/DD/HC/Additional Content/0,4054,676,00.html

It's all up to you exactly how careful you want to be and how badly you want to prevent pregnancy. Many people use the condom alone as their sole method of birth control with no problems at all. So if you use those two methods, you should be fine, even if you don't become pros at the natural family planning thing right away. However, if you really want to play it safe, you can look into a third method as well.