Building fault tolerant systems is a whole field, you are going to need to do a lot of study. It certainly touches on development methodologies such as XP, practices such as well-defined requirements and unit testing, use redundant hardware and power, use watchdog processes and well-defined error modes (with a catch-all well-defined way of crashing that won't kill your system) at all times, and also hire professionals. You still aren't going to be able to build something for aviation or a power plant with that though. For that you are going to need to make another quantum leap. It costs a lot of money and painstaking attention to make things that good. You might consider studying embedded device development paradigms too.

You may get some good responses here but I also recommend going to slashdot.org, where this sort of thing has been asked lots of time in the past (search for it). Or if you can't find it, post an Ask Slashdot. You will get a lot of replies.