The first thing I wanted to point out is a common mistake a few people make. One guy posted a picture similar to this one:
Right so with that aside, what can you do with mid/side eqing. Well here is a few ways in which I use it.
1. Clearing our side information from the low frequencies.
2. Dialling in some more 'side' information in the higher frequencies, and taking some of the mids away. Again this isn't essential but little subtle changes can make a big difference. On instruments which are higher mid & top end heavy, I like to apply a little high shelf but only to the 'side' band. Remember a lot of your instruments that contain high information will already be wide........wide synths, wide reverbs, panned high hats etc So it may not be necessary to over do it. Similarly I like to apply a very subtly high shelf for my 'mids' band, but maybe take out a couple of dbs from 10Khz and above. This is also the case for mastering.....when it comes to my mastering stage I go into OZONE and dial in anything from half a db to 1db of 'mid' into my low end, and take out half a db or 1db of 'side' in my low end. Yes this is hardly audible, but all these additions make up the finish product.
3. Keeping only the side information on little ambiances and atmospheric FX. A lot of the time, I like to place little noises or FX into my track but at a really low volume. This could be anything from a fucking bird tweeting to a car horn to a crowd of people talking. Its merely to fill in some of the background and add some 'real' ambiance to the track. Lets say I use a recording of an outside environment.....just the wind or something. I like to make variances on this sound by completely clearing out the middle information. I will set a high pass on my mid band really high so everything in the low/mid range is cleared from the middle. In fact a lot of time I like to just get rid of most of the middle information and just keep the side info. I will also add other FX like a wide tremolo to make the sound pan side to side but there will be nothing present in the middle to clutter up the mix. We all know the middle is sacred ground for your kick and bass, so get busy on clearing it out......even your little ambiances can be pushed away from the centre.