In my experience, it's better to choose something specific to widen during mixing rather than trying to widen an entire mix. Having one or two parts in a track that seem far left and right gives the impression that the mix field is wide, and that can be done without damaging other balances...and without lowering center panned stuff. I've used the stereo program in the Lex M300, a SPL vitalizer, a Dolby 740 spectral eq, console polarity and eq (flipped polarity and reverse pan followed by eq to minimize cancellation), and other toys to tweak apparent width. Keyboard and guitar pads, Rhodes tracks and other kind of "transparent" sounds can be tweaked in very pleasant ways, adding width to a mix without screwing up the meat of the song.
-tom