No calculators, it's all to be done in your head (I do belive you get a bit of skratchpaper though but I'm not promisining this)
The key for me seems to be to learn the most common stuff in my head, division by 60 is common (divide by ten, one zero less and then by six). Also it helps to learn the most common stuff in your head, like if someone goes 180 knots that's 3nm/minute etc. So if they go 250 (common speed beneath 10 000 feet, or FL100) that's a bit more than 4nm/minute (since 240 knots=4 nm / minute).
And in the end, practice practice practice
