As an Audio Engineer, one of the main goals is
to make the various mix elements balanced...
And the successful final mix should 'translate' to as
many different playback situations as possible...
Easier typed than done...
This is why many engineers use (relatively)
inexpensive reference monitors to mix on...*
"If it sounds good on these, it'll sound good on anything..."
Conversely, as mentioned, mixing for 'nice home-stereo'
speakers or headphones will most often lead to results
that won't translate to any other playback system...
So when mixing, use headphones just to check on things
(great for catching unwanted clicks, pops, rumbles or
coughs, ahems, squeaky chairs & kik drum pedals, etc)...
Also as mentioned, check your current mixes on various
systems and make mental notes as to what to re-tweak...
After several attempts (and/or years), you'll eventually get it...
And maybe invest in a couple of mix monitors...
__________________________________
* Mastering Engineers on the other hand, will often
use a Speaker/Amp combo that could exceed
the cost of my entire (small, but mighty) studio...
Good luck,
Bob Phillips
20to20soundesign