But you can trust your pants! SHAKE EM!

You can't trust your ears on inaudible 30hz and below.

This is actually what I've been doing recently, but to simplify things, for me it really just comes down to trusting my ears and a constant A-B between my track and whatever reference I've been using.

This is a great thread. Some of my methods are reinforced here and I'm also learning some nice tricks.

Also, weed.

WALLOFTEXTWALLOFTEXT SORRI

try not to over think about numbers and technicalities. really on your ears much more. i say this from my own experiences of just fiddling and fiddling and then making it sound shit. a decent song will get you further than a decent mix down on a shit song ! one thing i do too much is make my song too busy with too many layers all mudding up frequencies.......like when im just hamming late at night, writing melodies....then its a cunt to mix it down nicely. so i try to make decisions when i set up the track, like what takes the prominent bass, a subby 808 kick, or a deep bassline. often i sidechain sub with a kick if its beefy (like dem 808's) and then maybe the midrange bass/low meoldy frequency's are chained to the sub also.   
also another nice method if you have volume issues somehow, drowned or peaked mid to highs....i like to just load up a multi band eq and go through all the freq's low to high with the level right up, its easy to hear where you've got frquencies that make your ears bleed at just a small boost. i dont know if thats correct way but it helps me identify where i have some over lapping frequencies or reverb peaks and shit.....its good to clear that up mixdown era before going into the mastering. some mastering tips - dont buy krk's ever haha cos i hate them colouring bass heaps of dung. (yes i have them) the yamaha monitors made SO SO ,uch difference to my mixdowns when i had them. its true what their tag is 'if it sounds good on these it does anywhere'. they re great. anyway............check for sure at high volume but also low, i read somewhere that even when low volume you should still be able to hear the seperate tracks. like play a shit mastered track low and all you can hear is the high melody and hi hat haha, also i like to check like others, on everything i can. shit mp3 headphones. i have some i check my track on the, the high is peaking, no bass......blamed the headphones...listened to dope stuff like ital tek or apparat on them......no peaks, can hear bass.......so its my mixdown that sucks. if you're that bothered do your mixdown with at least 5 or 6db headroom to work with on mastering. and pay someone to master it to be happiest ! you can get okay jobs done by semi pro's over interweb. my friend got an ep mastered and when i mix the tracks in the bar i play at, they bump the club hust like everything else. i heard the b4's and afters, the mixdown was nice, but the mastering job really spread it like nice warm butter and got a few more db's out of for sure, without brickwalling it to sound shit with a compressor on the master. I wouldnt advise a compressor on the main master either ! anyway i hate this shit too, we all do....im recording my synths to 4 track, back into pc now, some field recording and basically producing in a way i like...just low fi but still cleanish.......but the low fi elements i like, and stops me going insane trying to make an overclean mixdown. that over produced shit has no soul and is just shit !

If you've got a compressor in your chain (or any gain altering device that would saturate the signal), I advise using at least two EQs on the track. One at the very end like Danimal said to control the overall sound, and one at the very beginning. Reason for this being, anything that isn't a sub-oriented instrument like a sub bass of subby kick does not need any frequency action from like 60-80hz and below. Obviously, you'd cut out all the lows from around around that and below with a high pass or something. Tracks that aren't subby will still energy in those low sub areas, so cut them all out. Over time and multiple tracks the energy in that area will build on your master channel and will attribute to a good portion of the muddiness that you might hear in your master channel. So, leave the sub frequencies to the sub instruments.

However, you should be doing this initial sculpting with an EQ that is the very first thing in your chain, not the last. Why? Devices like overdrive, saturation, and compressors (esp with makeup gain) will actually boost all those frequencies including the areas you don't want. Some people actually suggest using an EQ after every gain device to keep it under control, like so:

EQ (cut the sub) -> Compressor -> EQ (cut the sub area again because the compressor will boost it back up) -> Saturation -> Final EQ (cut the sub yet again, but also this is where you do any creative EQing for the overall track sound)

i mastered a recent tune so badly that i think i made everyone on Inpuj throw up

well that's fine, music should be exhausting

subtractive mixing, yo! i only use small boosts for most things, most of the eq work is cutting holes for other instruments. i also try and avoid panning low frequencies at all although i've come to accept that this might just be a personal taste of mine!

and if you have to automate stuff just DO IT, don't set and forget

I normally just go for some light compression and limiting, try it out in the car, a friends PA, headphones on my MP3 player, the stereo in the living room.

I hate doing mixdowns, I always think my trax sound so damn quiet, even if they are peaking in the software I'm using - Cubase normally. Quite often I'll hear a track of mine elsewhere and suddenly the bass is really boomy and mushy, really annoying.

The fact that I'm actually able to hear bass through the headphones I'm using makes me want to say you did a good job. These things usually cut almost all bass instruments out. And the arrangement isn't half bad either.

Right, so I tried to put the lessons to use. If you can, ignore the composition- I did that mostly to make the task more difficult.
http://chipmusic.org/µb/music/milgrams-cat

It would be cool if you could point out any errors or dodgy mixing.

Also, kudos if you managed to get through the whole thing

practice.

And make the EQ the very last thing in your chain, except for maybe a final limiter to catch all those snaggle-clips

That's good advice. I've actually done this in the past, when I mixed Sievert's album to make sure the mix is consistent.
What's your view on mix EQ vs master EQ? It's a lot easier to EQ the master if you go for a specific spectrum, but I guess making sure the mix is already good is the better way.

Currently I use 3 band EQ on each channel, and a 16 band EQ on master. I have difficulties controlling the sound that way, especially when compression is involved. Any advice on this?

yeah, a lot of referencing with commercial mixes you admire can go a long way. 

Listen to a lot of stuff back to back, do a lot of quick checking back and forth and try to notice what's different about their mix from yours