For mixing I use speakers and headphones that do very little unnatural treble and bass boosting so I'm hearing an accurate representation of what is really happening across the frequency spectrum.
Recommended mixing gear:
1) Flat medium sized powered studio monitor speakers that are decoupled from your desk with foam or stands.
2) A properly adjusted subwoofer and crossover set-up that you can easily switch On or Off.
3) Flat headphones that you can fine tune your mixes with.
These are my favourite "flat" mixing headphones:
These will probably be the next flat headphones I purchase: