This is a huge issue with myriad variables, but I'll touch on a main point or two
Firstly, don't beat yourself up.
Without mastering, most all recordings will suffer from variations in playback scenarios. Even once mastered, there are natural changes in tonal colour from headphone to headphone and speaker to speaker. The aim is to get is as solid as possible on as many sources as possible, while sounding kick arse on the high end.
Mastering is something I have learnt slowly (and I am still learning) over a decade and a half of academia and studio work. I have my own algorithms and methods (which alas, I am loathe to share exactingly... for fairly obvious reasons!)
Mastering is part art, part science.
The art is getting the initial mix right, and then applying the correct compressors, limitors, EQ, normalisation algorithms... each at the right stage....with the right settings. The science is understanding HOW these affect the sound wave in order to let the artistic decisions flourish as you wish. And of course to understand the dominent and sub-dominent sound frequencies of your mix, and the relative volume ratios of each frequency band area.
Should much of the above read like goobledigoob, fret not.
It isn't easy. I am still learning year on year, and improving (one would hope!) despite doing this for a living!
As I say....don't beat yourself up over it.
It may be easier to get as good as possible with your desk mixes, and then find a local mastering/mixing engineer who will work on your mixes. Those you really want to shine. Ask to hear his/her work. Then, try one song to see if you like the results. Yes, its a fee to swallow, but this is a specialised skill.
AWAY from mastering
-The comparison trick is vital. Have a folder loaded up with your favourite mixes of commercial songs with "similar" sounds. You can't match mastered with unmastered though. So be careful with pushing instruments that are "compressed" forward such as Bass drum, Bass, vocal presence. Don't over compensate, work within constraints
-Test on multiple systems.
-Mix to the song. Don't try to be something else
-Learn compression, and keep learning. BOYDIE has been on a compression bender of late, and is improving with each mix. Having sounds well compressed can have a greater impact on the mix feel, and will lesson variation across speakers/headphones. HOWEVER, beware over compression. Red peaks at the extreme; lifelessness to your dynamics are a risk with even a little too much
Lots to take in, I hope at least some of this helps
Small plug - Hey I earned it with the big ass post above!: I do mastering for external clients as well as my in house productions. PM me for details if interested.
www.riverwaystudios.co.uk
Plug over!