Ok this is a genereal problem for unexperienced clients or mixer engineers and they really need be educated in this regard. I'll split this problem in two category: ITB mixing and OTB
This problem it's for ITB guys but I saw it exceptional on OTB too.
Mastering sessions need headroom for dynamic processing and exceptional for EQ processing too in the add cases. For dynamic processing we realy need high RMS to Peak ratio level that's why it's important to handle compressions during the mixing stage. And NO, it's NOT a fad. I just realy want to do a good job for you and I you must help me to help you. Often time I get mixes with limiter seting on -6 dBFS because clients read some articles where they saw that must deliver mixes with peaks level - 6 dBFS. Yes -6 for peaks it's good and I can support one transient (eg. snare accents) that peak to -4dBFS but I realy can't support a mix with - 14 dbFS RMS and -6dBFS PEAK = 8 dBFS RMS/Peak ratio. So forget the -6 dBFS peak rule for mixes deliver and get this. If you mix in 0 VU seted on - 20 dBFS I guarantee you that your mix will stay ~ - 6 dBFS peaks. No, 0 VU it's not RMS. VU it's my only reference after my ears. Assuming that you understand this lets starting:

FIRST THING: TURN UP YOUR's MONITORS LEVEL

ITB

~ 5 years ago PSP come's out with a great FREE plugin. PSP Vintage Meter. You can download it for free on the PSP official website http://www.pspaudioware.com/
How can you stay in 0 VU ? Simple, put this plug on the POST FADER Ins from your DAW and set it like this:
VU integr. time = 300ms
0 VU refer. level = -20 dBFS

From this point you can start your mix. Stay on ~ 0 VU with your mix and forget about the peaks. Don't starting mixing first element (eg. kick drum) on 0 VU because you'll get more energy during the others tracks mix and you'll easy reach from 0 VU. I can't tell you a recipe because it's program dependent. Experience will say you.

OTB

Are few OTB users that don't know this but just in case. If your console or summing mixer don't have a VU, get one. You can find on ebay. Then calibrated your 0 VU on -20 dBFS at 1Khz on your VU meter. How to calibrate your VU ? Get a sine wave generator. For example Spectra Foo. Generate a continous 1Khz sine with -20dBFS level and for this level tune your tracks trims to point 0 VU on your meter. This is tricky if you mixdow on TAPE. But if you mixdown on tape on these days I assuming that you realy know what you're doing.

One thing and I'll finish this post. Many of you want to get a punchy mix. You must know how to produce a punchy mix. It's not that simple that you put a 4000 bus compressor on master and you'll get a punchy mix. It's more:
- how drums are played
- how groovey feeling have your bass player or you in case you do sample based project.
- how compression raports are made during the mix. You can't do a punchy mix with compression on every track. To get loud sensation your ears need to have one to compare, and if all your tracks have compression and are loud your ears can't get a punchy sensation.

I allways said no rule in music but you really need to stay on objective vision.

cheers