Getting width (and depth) in a mix means providing the listener with audible cues that mimic the types of cues we hear in real life that help us determine which direction a sound is coming from. Figure out how spatial perception works and you can replicate the effect in your mixes, giving them a sense of spaciousness.
Caveat: some of those cues are dangerous to duplicate in a mix. Much of our spatial perception comes from phase differences between what each ear picks up. You can simulate that electronically, but you can also totally hose your mono compatibility in the process. I would generally advise against plugins that use phase tricks to widen stereo tracks or full mixes. [But I can recommend a pretty good one if you insist: it's called
Hbasm Stereoizer. There is a free version and a very cheap ($19) version.]
The good news is that you don't need a fancy plugin to get width, and you can achieve it without losing mono compatibility. The key to width is detecting some kind of
difference between what the left and right ears hear. Phase is just one of the differences we can sense.
A mono track such as a lead vocal cannot be widened, only panned. However, if the vocal is double-tracked - which as Matt notes above, is routine - the two tracks can be panned slightly apart. Because each track sounds a little different from the other, the listener's brain interprets it as width. A much less-effective technique is to use a delayed copy of a mono track, or a chorus plugin, or something like Waves Doubler. All of these yield a similar spreading effect, but nothing beats real double-tracking.
Reverb and delay are crucial to depth, but they can also play a role in widening. Subtle ping-pong delays are very effective, especially on lead instruments such as guitar and saxophone. Panning an instrument's reverb to the opposite side is an easy trick for widening. If you use two rhythm guitars panned hard left and right, try cross-panning their respective reverbs. (That technique ain't for yer metal tunes, though; that genre's got carved-in-stone rules you don't mess with.)
In addition to cross-panning instruments and reverbs, I'm also a fan of movement in reverbs. PerfectSpace has a cool feature that allows you to apply left/right movement to the reverb tail. [And I'll even share one of my secret techniques here (don't tell anybody): a nifty free plugin called
Pancake from Cableguys. Put it on your reverb bus.]
Another big area for width-enhancement is M/S processing with equalizers and compressors. A standard trick is to boost highs on the Side component. This works because our directional sense is based on high frequencies (over ~1KHz), so by emphasizing highs in the Side component we exaggerate any differences between left and right channels - and L/R
differences is the name of the game!
Similarly, compressors that let you unlink left and right channels also emphasize L/R differences. For example, my favorite limiter (FabFilter Pro-L) lets you unlink transients and/or release times. This allows smaller, unlimited peaks to poke out independently on each side, which promotes L/R differences.
Some compressors also support M/S processing, allowing for separate treatment of Mid and Side components. Having a longer attack time on the sides emphasizes transients, which again exaggerates L/R differences and gives a greater sense of width. I'll often compress the Mid portion more than the sides, which lets you fatten a track without losing side dynamics.
Gosh, there must be a gazillion tricks for making mixes wider. When experimenting, just remember that the key to width is differences between left and right channels. Anything that encourages
differences will help make for a wider mix.
Uh, I guess that's enough rambling...I know I really shouldn't post so soon after burning a doob.