Fabfilter have produced a series of three great videos produced by Dan Worrall titled How To Mix In Stereo Without Sucking In Mono.
The videos include a great rundown on some of the pitfalls of mono compatibility, some neat ideas around tuning your Haas delays, and using filters and EQ to manipulate phase to make space in a mono mixdown. Really good stuff, and recommended viewing for anyone who mixes audio. Check it out at: