I've made a bunch of flat & V-springs for old guns. I agree with most of the above, except that when I temper a spring, I use nitre blueing salts.
I heat the salts to 575 degrees F, and dunk the spring in for several minutes. You should come out with a deep blue color. I like tempering in nitre salts because I get a very uniform tempering. You could also use a burn-out oven, but that will take more time.
Also, I polish the spring steel lengthwise. I tend to taper my springs both in width and thickness, so as to get the force to be spread across the spring's length, rather than concentrated in a small area of the spring.
Old time gunsmiths used to prefer to quench in Sperm Whale oil, and then use Sperm Whale oil to temper (per Bobby's instructions of using a snuff can of oil above) but that oil is now rather difficult to find.