Hey JOHN
There is soooooooooo much to say
I think it would be easiest to "show" you
If you want to PM me with a download link for your mix (without any of your mastering) I would happily have a crack at mastering the song for you and giving you some specific feedback on the mix and what I have done with the mastering (assuming you like it)
This would be completely free of charge by the way
The art of mixing is to not just get a good balance of instrument volumes and frequencies (and a gazillion other things) but to get a mix that "translates" well to other systems - eg ear buds, hi-fi systems, mobile phones, car stereos, club systems etc.
All of the "tips and tricks" to achieve this are the same general "rules" for mixing
The most important part is probably your mixing environment/equipment
Eg - if your speakers emphasise the bass frequencies (like domestic hi-fi systems tend to do) and your room that you mix in also emphasise lower frequencies you will naturally mix the bass quite low as you will subconsciously compensate for your environment
If you now play on another system, in another room, without these bass anomalies your mix will sound very light on bass
This is why it is worth spending time "getting to know" your speakers and environment by playing some commercial mixes you are familiar with and notice the differences between systems
Sticking to bass for another example...
If you know people are going to listen on earbuds and smaller speakers (eg iPhones or iPad speakers) you know that these system will not reproduce a heavy bass sound
It might, therefore, be a good idea to copy your kick drum and add a frequency boost in the lower mids (or if you are using midi drums play it with a low Tom sound) so that you will still clearly hear the "kick" pattern on smaller speakers