Sound treatment bass traps and tri-traps are in. Trying to decide whether it sounds good or it's just the placebo effect. I thought it sounded shit at first, but then I stopped listening to Placebo and it sounded pro.
(Drum) Room mics are in. Boost the hi and low end with a Pultec style EQ, take around 3db off with a compressor and then squash with a limiter until it sounds nice and washy. Put the fader on -infinity and raise until it bites with the guitars and adds some depth to your drums and mix. Add a parametric EQ and lo pass filter if you have a lot of other bass stuff going on in the mix which you'd prefer not to muddy up, you may however like what the room mic low end does to your mix.
Mixing has 3 stages:
1. Get all the instruments to sound good on their own. Remove any harshness or boxiness, make sure things sound big.
2. Get everything to sound good together - Balance your mix. Control low end on certain elements e.g. kick, bass, guitars if appropriate. Revisiting step 1 will probably be necessary. Make some surgical cuts if you think it needs it but be careful, a lot of noobs destroy their mixes this way. Bass is generally okay to perform severe surgery on though. Don't be scared, it can handle it.
3. Personal preference. In reality you don't want a balanced mix. Songs are characterised by what you choose to feature and hold back. I like loud kicks to drive my songs. I like my vocals to sit on top of everything. Some people like a super prominent snare. This is where your individuality as a mixing engineer shines through and what mixing trends and fashions tend to influence.
Extreme high end and Extreme low end. Aforementioned room mic trick helps with high end especially.