[bscphil]

Personally I think the usual issue with the standard conversion is doing 9/5, but it actually isn't that bad. The way I convert is

C * 2 - C / 5 + 32, where I just round C to the nearest multiple of 5 before dividing to make it easy.

24C times 2 is 48, 24 / 5 ~= 5, so 43 + 32 = 75. Off by less than a half a degree, which (thinking about it for 15 seconds or so) I think is guaranteed under this system.

The most important thing is that this is easy to remember because it's basically just the real conversion, but also easy to do in your head.

[dminor]

It's even easier if you consider that C/5 is 1/10 of 2C. So, just double C and then subtract 10% of the result.

[mrep]

Kind of like how I calculate the tip: move the decimal place 1 over, double it and then round down.