Three things...
A KRK 10" sub. This seemingly simple one item helped cut my final mix time down by 50%. Before I got it, it was listen to monitors, put headphones on and double check, make a wave file, go to my stereo and check, go to the car and check, do-over. Its been weeks since I did anything over because of incorrect bass mixes.
New headphones. Beyerdynamic DT880's. They have helped me really HEAR the bass drum and bass guitar frequency components better, along with detail stereo imaging components of the mix. It may sound a bit off, but now that I can hear more detail in the soundstage, I am using FAR less effects, and the music is translating exactly as I imagined it would while I was tracking.
This last one doesn't seem to be studio oriented, but it has cut my setup time drastically - to a few minutes, along with simplifying setup and dialing in sounds I want as fast as I can think of them. Its a new front end rack for my performance equipment that includes a voodoo Labs GCX switch and Ground Control unit. I built out a new rack system (for both performance and studio in mind) including a rack drawer for my foot pedals between the radio and the front end for the amps, added a new rack mount Lexicon and Eventide with the old SPX1000 for the time based preamp loops on the amps, and a new rack mount tuner. I used the best noise free cables I could get. I know, for most, printing effects doesn't sound like the smart thing, but I have been writing and recording new country for a while. I can hear what I want in the sound before I commit to a track. In the mix down, if something isn't right, I just do over and I LOVE IT. What's better than having your axe in your hands creating in the studio or performing live? :o) I have each effect set to a different footswitch for a total of 7 (I have one button set as a kill switch), and every combination of effects sounds I use programmed into the user definable bank segments of each effect unit, which I rotate through using up\down and the bottom 4 buttons. Since I perform far more than I record, I roll the stuff outa the truck, into my little room, plug in the amps, open the drawer to the pedals, mic / DI the amps and GO.. You could also say that it has made the 'soundscape invention' of what I do a heck of a lot easier too. Out of the three, voodoo takes the Cake.
LL