Vastman
Go to sonarworks and pick up their headphone calibration system.  They have modeled a long list of headphones and their system works wonderfully on my KRK 8400's... you have many options, as you'll see once you arrive at their site.

I was one of the many pushing Sonarworks to include the KRK headphones. I exchanged many emails with them and they did indeed come through. Been using Sonarworks ever since.  My mixes translate much better now on different systems. I never realized how much high end those headphones added until Sonarworks. My mixes always sounded dull as a result.
 
Highly recommended!

I don't use headphones for mixing/mastering.  I use studio monitors.  But, I was having problems with too much bass on my mixes with Samson Resolv 6.5a studio monitor speakers. I had to guess and remix and remix to get it right.   I bought new JBL LSR308's and no more "too much bass" in my mixes.  
 
I specifically bought the 8" LSR308's because of the bass problem.

Just from a practical point of view you probably need to Hpf everything except kick floor tom and bass and even those will probably stand some really low frequency filtering.
Then do the room or headphone eq trick

I struggle with this too, and the ONLY way I've been able to get my mixes close to where they should be is to use a reference mix.  I mix in headphones primarily, because my studio is an untreated spare bedroom with all kinds of bass build-up and reflections, etc.
 
When I feel my mix is done, I export it as a wave file and import it into a new Sonar project along with the reference track.  (I have a song in the Songs Forum I completed last week using Zac Brown Band's "Toes" as a reference.)  I apply mastering fx and repeatedly compare my track to the reference, especially listening for the low end and that balance of instruments to vocals, etc.  Usually I find problems that I have to go back and fix in the mix.  I repeat the process until I feel good about my track.
 
After I export my master with the fx and maximizer/limiter applied, I listen to both the reference and my master on my iPhone and in the car with the best sound system.  I usually listen to a few other professional songs too, to compare volumes and overall sound.

Thanks a bunch, guys!  It sounds like the best way to get good mixes is to mix on good speakers.  Like fireberd, I have some Samson Resolv 6.5a monitors, but they've been sitting in their box for about 5 years now, mostly because we've been renting houses with thin walls and no sense of privacy (having 2 small kids doesn't help since they'd bang on the door the instant they hear sound emerging from my office).  Perhaps when we get our own house, and the kids get older, I'll be able to break the dependency on headphones but until then I'm afraid I'm stuck.
 
The next best approach seems to be to mentally compensate, by using reference mixes, and trial-and-error.  This, of course, requires time, patience, and mental energy, and, well, did I mention the kids?  I'd prefer a solution which doesn't require conscious, deliberate compensation. 
 
I was hoping for some sort of plugin or bus setup which would let me adjust my monitor mix, without affecting the mixdown, and without my having to remember to turn off the monitor plugin/bus prior to mixdown.
 
Is there any way to route different audio to monitors than what would get mixed down?  Or is Sonar pretty much WYHIWYG (what you hear is what you get)?

Tripecac
Is there any way to route different audio to monitors than what would get mixed down?  Or is Sonar pretty much WYHIWYG (what you hear is what you get)?

It depends on your interface. If you have multiple outputs, you can set up a separate headphone cue mix, as you would if for example the bass player wanted to hear more drums in the mix while overdubbing.

i think most 'non-pro' mixer systems deal with it... 
hi pass about everything.  throw a multi-band compressor on the mix and solo the low bands.  i added a sub for mixing.  i still have resonant issues on home systems compared to pro mixes.  I use Sennheiser and Bose headphones (very different sound), as well as the other typical sources.
since you brought it up, i found this donation-ware that I'm going to look into:
http://www.roomeqwizard.com/
if anyone has used it, pls post your results.

For me, a good mix is a good mix, whether it's on headphones, monitors, a hi-fi, the car, those Apple earbuds, whatever.  I used to have the same problem - bass heavy mixes.  I learned that my monitors don't have much bass, so I do most of my mixing on them, then switch to my computer speakers, which have a subwoofer, to manage the bass.  If the bass sounds a little weak on my monitors, I know it's probably OK in the real world.  Then I listen back in the car, on my iPod, on my little Bose bluetooth speaker and make notes about how they sound on each.
 
The other thing that helps immensely is a high-pass filter on most of your tracks.  Kick and bass guitar have to have special attention.

How do you know which is right?  Home and car stereos often jack up the bass, and earbuds are all over the place.  The barnds all sound different and can even change by how they fit in your ears..
 
If you have good quality near-field monitors and listen at a moderate level, I would think you could get pretty close unless your room is really strange.

konradh
How do you know which is right?  Home and car stereos often jack up the bass, and earbuds are all over the place.  The barnds all sound different and can even change by how they fit in your ears..
 

Mostly I judge based on my monitors and secondly, my PC speakers.  If I get it sounding good on them, it seems to translate pretty well anywhere else.