The most important and useful as it makes C-to-F conversions much easier.

add 40--convert (9/5 or 5/9) -- subtract 40

or, as I tend to use,

add 40--convert (2 or 1/2) -- subtract 40.

How does that crude approximation make it any easier?

I am pretty fluent in conversion just by remembering that 10 Celsius degrees are equivalent to 18 Fahrenheit degrees, then practicing counting by 10s:

-4°F, 14°F, 32°F, 50°F, 68°F, 86°F, 104°F

Once you have those under your belt you can figure most earthly air temperatures pretty quickly and exactly.