XKCD have posted this, slightly NSFW, guide to thinking Metric (click to embiggen). It's pretty good and is the kind of mental conversion I do when thinking Imperial.
In the USA I convert temperate based on what a Guppy likes. I kept tropical fish as a kid and most references quoted Fahrenheit so I know 77ºF is 25ºc (Happy Guppy/Robin) and 32ºF is 0ºc (Dead Guppy/Cold Robin)! Every other Fahrenheit temperature is mentally related to the Guppyness scale!
