a standard..
I think that everybody that produces looks at the "wave" he created before bouncing it into an mp3 (at least, I do)
then you can see what volume your tune is.., I try to approach 0 dB, with (almost) no spikes above 0 dB (they are cutoff anyway = reducing sound quality)

Very true (Well, I dont look at the wave myself, I just keep an eye on my VU meters, and most of the time i have a limiter to take out the +0 peaks on the master)... but 2 tunes can look equally loud in de soundwave, while one could sound alot louder...

Mid range boosting. Human ears are optimised to hear midrange. So if you boost midrange through the roof, your tune 'sounds' louder. If it sounds better, is another question altogether

Best you can do if you play out mp3's, is normalizing them before you put them in your crates. Most tunes you buy on the internet will already be normalized as part of the mastering. Dubs and mp3's made by producers themselves, might get a little added gain from normalizing them.
btw: that gain-knob on your mixer serves a purpose!

All the tunes I buy I run through a program called "Platinum Notes". This is a piece of software that "heals" your music. By restoring cut off peaks & other stuff. It also normalizes all your mp3's to a certain db. So I seldom have the issue that one tracks is louder than another. I do notice an increase in soundquality of the tunes. Although very minimal when buying tunes online, but it can do wonders for tracks that I record myself from a piece of vinyl.

http://www.platinumnotes.com/

Check it on the spectrum analizer (or just on visual EQ): you'll see that your "quieter" tune will fill the spectrum less nicely than the loud one.