Good question!  I deal with this on a daily basis.

Even though your recording is peaking at 0 dB, the reason it sounds quiet when compared to other commercial recordings is because it has a lower RMS level.  RMS means "average volume"--that's what makes a recording seem loud vs. not loud.

The best way to address this is in the mix, but there if the mixing is complete, you should try limiting the peaks.  Set a limiter to 10:1 with the attack almost all the way up and the release time just a little further back.  Set the threshold so that it reduces the peaks from between 2 to 6 dB (this is a matter of taste).  Then, you should be able to turn up your track to 0.  When you do, the average volume will be louder. 

Most "loud" recordings have an RMS of over -10dB.  I prefer between -15dB and -13.  Making your disc too loud will seriously date your recording. 

If you need any more help with this, feel free to email me at studio@mikegarrigan.com  I'd be happy to fix this for you.

If that doesn't work, and you can recall the mixing session, there are also some simple things that can help make the mix louder.

First, if the mix is peaking at 0dB and the mix isn't very loud, that means that some instrument or part is too loud in relation to the rest.  It's dynamics need to be tamed a bit.  The frustrating thing is that it could be anything--a vocal, a drum hit, a bass pop.  The key to honing in on this is to check the meters and see what's up.

Second, the peaks could be caused by redundant information in the lows.  You'll want to be very careful with this technique because doing too much of it will cause your mix to become thin.  First, try using a hi-pass filter, set to 40 Hz on the bass.  The kick won't ususually have enough information at this frequency, but that all depends on how it was tracked.  Next, set a similar high pass for the overheads but set it to 150 Hz.  If the drums were tracked right, this should eliminate doubleing up in the lows, which might be causing peaks that you can't hear. 

Third, vocals can tend to get a little crazy sometimes.  I often limit them fairly liberally in tracking and then do a second pass of compression in mix down.  This allows everything to be heard and the loud stuff to get turned down.  Also, if you have the luxury of time, you can do some automation in your DAW.  This is often preferred over compression. 

Good luck with your project.