If so, what plug-ins are you using? And how are you using them?

I like to widen the mid range *only*. Varies according to the song, but somewhere between about 750hz anywhere up to 2k. Can really aid clarity in a pretty transparent way. Low end I keep as close to mono as possible, and high end I tend to leave however it is in the mix. I use Ozone for this. Very easy to break the frequency range into bands and tweak each band differently.
 
Ideally, mind you, I'm not a mastering engineer, and I always advise bands I'm working with to get a real mastering engineer to do it if they can. But I've got into the habit of delivering a master at least good enough for indie band soundcloud / bandcamp purposes, in case they don't have the budget for that. I also always deliver the pre-mastered file. For anything with a budget against it, I just do the pre-master.
 
So my advice is not worth much more than any keen amateur, in mastering terms, really. Have yourself a pinch of salt with it.

 I'm not a mastering engineer either. But for a nice DIY polish on a mix,Ozone 5's "excite and widen" CD mastering preset is an awesome starting point.  

Sounds like it is time for a real mastering engineers input. Once you start putting mix integrity first it is simple, you just dont do it. Whatever is going on in order to widen something is not going to be good for the mix. It might sound cool but with careful listening you will find other things are sacrificed in order to do it.
 
Better to go back to your mix and see what you want widened and do it there. Still by far the best place to do it.

I hate artificial widening gimmicks. The width has to be in the mix to begin with. However, I won't apologize for enhancing the width that's already there. My favorite tools for that - on the master bus - are Pro-C and Pro-Q in M/S mode, and Pro-L with stereo unlinking.

Listen to Jeff and Bit; they speak wisdom.

Especially this: "Better to go back to your mix and see what you want widened and do it there. Still by far the best place to do it."

Be good to your mastering engineer; don't make him or her fix what you did wrong. That's a waste of their talent. Do everything right, and let them put a cherry on top of it.
 

I've found widening useful if the voice is mixed too high and is panned center, as widening the mix reduces the center emphasis somewhat. However this also alters other elements in the mix so it may or may not work. It's a "fixit" technique.
 
If the mix is good, widening will likely upset the balance that made it a good mix in the first place. However if the mix has problems, widening may help counteract those problems if the way it unbalances the mix pushes it in a better direction. This depends on luck more than anything else, as the results of widening are unpredictable. 

It can be surprising how few things in your mix you might actually want wide. Find out what it (they) is (are) and work on that (them) only.  Leave most other stuff where it is.
 
That is if you can work on your mix of course.  If mastering only and you don't have that ability then well widening may not be such a good thing after all. I have rarely heard widening a whole mix sound much good. (It does depend on the music of course)
 
Craig wouldn't another way to reduce vocals panned centre might be to convert a mix to M/S and maybe EQ the M component such that frequencies within a certain range (vocal range) only get lowered once the M/S signal is put back to stereo. I am sure there is a better way to do it rather than widening the image. Only just some ideas of course.