It very much depends on what you are doing. How many errors per KLOC can you deal with? A website isn't going to have the same requirements that NASA does. On the one end, try acceptance testing and user testing. Its cheap and fast. On the other end, provably bug free code is difficult (not quite impossible http://www.ece.cmu.edu/~koopman/ballista/index.html) but requires better documentation than anyone should ever write.