I'm probably going to say lots that's already been mentioned. The secret to excellent mixes, is not what gear you have (a £50 guitar through a £20 amp isn't going to help things much) its all down to arrangement, the players and good decisions in pre production. Put this together with a great mix engineer in a great room with great gear and everything should sound good. Don't go chasing the next best marketed plugin (been there done that) people were making great sounding records on sub par gear to what we have now a days many years ago. I'm still on pro tools 10 HD and have been for years, haven't brought many new plugins for years either. I've go my arsenal of gear that I'm familiar with and know how to use and I'm sticking with it. A well treated room will help your mixes so much, before I mixed in a treated room my mixes where always lacking bass when played anywhere else. The room was adding bass to my mixes which I thought was there in the recording. My advice is get a well treated room, some fairly good studio monitors, a range of plugins and practise, practise, practise. Those great sounding mixes will come trust me.
Rest assured guys, the idea of getting new plugins has flown out of the window!
I find that my bass decisions are alright, but my mid range always sound very ill-defined. Any suggestions on what I can do to "sharpen" it? It sounds great on my monitors but sounds muddy to me when I audition on other devices.