Use an SSL Quadcomp style compressor on the master bus, no more than 4db compression for 'glue'. Ratio set to 2. The release can be auto, if your auto is worth a shit. Attack set to whatever sounds good. 30ms will allow the transients through and make things spikier. .1ms makes things ultra squishy, smooth and maybe a little too creamy with no real crispy transients an more. I'm currently liking 3ms as a good compromise. Use your ears. What could and should be are very subjective. Again, nothing should be louder at this point. It's just peak slapping and making it creamy and 'together' sounding.
Mix through this comp it and don't listen to the whiners. People sat behind big desks do this and they make records that sell more than a few hundred. The ME complains if it's overdone, but the mix engineers are in control of the sound of the overall vision of the mix. Just slap the peaks, don't destroy them and the ME might even be happy. Periodically check to make sure you're not squashing too much and adjust your threshold if you're going over.
Again, you're not making it louder at this point, you're making it a little flatter and more together. Turn the volume down until you can BARELY hear it. Make individual channel adjustments at this low, low level. The bass should be a nice, thickish carpet. You're aiming for ear satisfaction at this point. A nice flat response at this super low volume level. A carpet of sound, lovingly caressing your silly balls. If you turn it up from time to time, it will still sound fucking good. Conversely, if you turn down a mix that sounded good loud, more often than not, you're going to have holes in it. If not, well fucking done!
If you turn the comp off momentarily, if you're doing it properly, you won't hear a world of difference. The balance should still sound pleasant enough. They should simply sound more 'live' than a recording. Think of it as listening to the live version, rather than the studio version. If you've made a pleasing mix, when playing it really low, you should feel the urge to turn it UP with your magic wrist. It's a strange thing, but try it. Try listening to various good mixes really low. Some will have a nice thick production that makes you want to turn the fucker right up because you want to be surrounded with it. Other mixes might seem thin and holey and you won't want to turn those up.
Make it sound as good as possible. Print it. Don't listen to it for a while. Rest the ears.
1 week later, listen to the file. (It should be a 24bit wav file, no dithering)
Listen to it super low and at a moderate level. Still sound good? No? Go back and make individual channel adjustments, OR try and play ME, but understand they're magical voodoo people with golden ears.
Throw a multiband comp on it along with a nice EQ and try and get it sounding full/flat at low volume. This will yield different results from messing with individual channels, obviously. You're messing with the hole mix like it's a piece of clay and you're probably fucking things up if you don't know what you're doing. At worst, this is 'rescue', at best it's final tonal balance. Or something. Listen quietly and you might make some magic happen. You have less controls here than in the full mix, so you can immediately fill the holes in as you listen.
Happy? Does it sound like a thick lovely carpet of sound when played low? Now, make it loud. Throw it through a limiter. An evil fucking music squashing limiter. After messing with a bunch of other limiters, I'm pretty happy with UAD's precision limiter. Lucky fucking me. It seems that I can just squish it up until it's about -12dB RMS and it doesn't lose much of the original character. -12dB seems to be the best place to squash anything to my ears, anyway. Any more seems to just kill it completely and make it sound fucking horrible, any less seems to be too 'quiet' played next to commercial releases. It seems to be the happy medium after analysing a lot of commercial releases, the better sounding ones seemed to hover around this mark, too.
Listen really quietly. Still sound good? It should do. With the precision limiter, the tonal balance doesn't seem to be too affected. If it does sound way different, can you rescue it here with your pretend ME hat on, or are you going to have to go back and do it properly from the mix end?
That's where I'm at in the ghetto 'mastering' stage. A marked improvement from whacking it through a sound wrecking crush plugin, which is unfortunately what I've done with everything people have heard up until now.
Listening REALLY fucking quietly seems to be the key.
I wouldn't do this for an actual release, though. I always send it to somebody who knows what they're fucking doing.
