How loud do you run your mixes pre-mastering?

You dont have to switch it off forever, just turn it off temporarily, then control+select all the track channels and bring them all down unison until it's peakiing at most -6dB, some people even go for -10 or even -12dB...then turn the MB comp back on and you'll be fine and have closer to commercial volume.

You can always turn down the reference music to match your tracks as well.

I personally prefer to route them all to an empty bus that I route to the master bus and lower that by 12dB. That way your mix + automation doesn't get fucked up.

thanks guys, a lot of usefull info in here. so I'm attacking the mix a bit differently now. It's peaking at around -6, but the average RMS is somewhere around the -15 to -18 range. Is that normal or should I be trying to get the average higher by means of compression or something?

Considering that my mixes hit about the same without master bus processing, it's quite normal.
 
thanks guys, a lot of usefull info in here. so I'm attacking the mix a bit differently now. It's peaking at around -6, but the average RMS is somewhere around the -15 to -18 range. Is that normal or should I be trying to get the average higher by means of compression or something?

Don't think about RMS when mixing. Just leave enough headroom and mix away trusting your ears, and you're good :)