Hey man,
Maybe look into some 'grot box' speakers. There is a ton of literature on this stuff...but most learn trail and error...or posting on forums
In a nut shell...with the mp3 revolution...most people don't listen to music on anything near the qualities of your monitors...plus...if its club music...those systems are often mono with massive amounts of bass. Additionally, the acoustic environment you mix can make a HUGE difference in what you hear and how you mix...it can literally drive you mad.
in any case...the two biggest things you can do is invest (cheap) in some sort of sound attenutation material...it doesn't have to be much..and don't over do it...but if what you are hearing ain't whats actually recorded...you mixes will be inaccurate and you'll be chasing your tail. Again...lots of stuff about this...check out sound on sound magazine.
Secondly...those grot box speakers. they're usually cheapy's...you can use a cheap boom box...aura tones...pyramids...even the venerable NS-10's are somewhat grot...the idea is the use your krk's for fine adjustments...but you use something cheap to get a better idea of what your mixes will sound like through the system which it will probably be listened through. I've seen lots of pro's hook up a cheap sony boom box with heavy bass and just monitor through it real quick to make sure the definition is there and the bass ain't gonna blow out the speaker. They keep it hooked up next to there real monitors and use it frequently for a change in perspective.
In the end...I agree...monitor through as many pairs of speakers as you can...including headphones...and several different pairs. Learn you monitors and room over time...you can't expect perfection early on unless you get lucky...each mix will get better as you learn how it translates to that car stereo.
Finally, as a last resort...burn a cd and listen to it your car...that the old tried and true method...not very elegant...but I'm sure all mix engineers have used this technique at some point in there career.
good luck.