So I always make a cd and drive around with it in order to deal with this.
lol,,,the things we do to achieve a good mix,,,,, and thats just it,, try everything ,,develop an ear for the sound ya looking for ,, An older method of mastering and (I'm sure some of the 'best" will deny) is to use an in-expensive stereo system,,just an amp and a pair of speakers or even try your computer speakers, I've produced some good mixes using a pair of 10$ altec lansings,listening at a "low level"..... one day! it will, just happen for ya,,,
Also,, A major consideration is who is the final mix for ,,you or a client ?
You will have to adjust your mixes accordingly for each client ,, thats alot of fun