7. If you're playing music through your phone, adjust the sound settings
Price: Free, but there are some superior options for a few bucks
Kick it up another notch by adjusting the equalizer on your phone based on what type of music you're listening to. Just go to Settings > Music > EQ and play around with what sounds best. Note that this only works for music played through Apple's proper Music app (sorry Spotifyers).
There are also a handful of apps you can try, like Car Vitalizer for iOS that adjusts the timing of each audio channel so everything hits your ear at the exact right time when you’re in the driver’s seat.
8. Or skip the phone and use a thumb drive
Price: Free, assuming you've got a spare laying around
As with any type of system, you want as few interferences as possible between the sound file and your speakers. Your phone has a built-in digital audio converter, or DAC. If your car's stereo is already pretty good to begin with, its DAC is likely better than your phone's, so you'll get a more clear sound if you put your music on a thumb drive and simply connect via USB.