A key ingredient of a modern mix is that it not only registers a certain value on the RMS meter, but it also feels loud. A mix can be compressed only so much before it starts to sound flat, lifeless or pumping. However, there are certain things you can do to make the mix bus compression work for you, not against you.
Keep the low-end tidy
Untamed low end not only makes the mix sound unprofessional and muddy, but it can also make the mix bus compression do more harm than good. High-pass filtering can free up a lot of valuable headroom in a mix, even though it may not be too audible of an effect.
Kick drum and bass fundamentals should each live in a frequency space of their own, say, for example, 60 and 80 Hz - whichever way you choose. Once the low end is under control and headroom is freed up, the mix bus compressor will react to the song in a musical and rhythmic way that serves the song.
Clipping is your friend
One of the most frustrating things in a mix is a stray drum transient peak that can makes your mix bus compressor pump when it comes in just a little too hot. Drum clipping is a very efficient solution to this problem, especially on snare drum and toms. Instead of using a limiter that pushes down the signal that crosses the threshold, a clipper simply chops off the peaks as necessary, therefore making sure that the signals stay consistent in a very transparent way.
Clipping makes gain staging easy and predictable, so that the drums don't put unnecessary strain on the mix bus compressor. If you're in for a more aggressive drum sound, you can also clip the drum bus as a whole to make the sound denser and loud without destroying the mix bus.
Mix bus compression
Before getting into the specific details, it must be said that mix bus compression should be viewed as just a tool for a specific job, not as a magic bullet to salvage a poorly balanced mix. The most popular compressor choice for mix bus application is a VCA-style compressor like the SSL G series or API 2500 series, for example.
A good place to start for a loud modern mix would be to set the attack to be fairly slow (20-30ms) and the release to be fast (0.1s), at 1.5:1 to 2:1 ratio. Aim for 2-3 dB of gain reduction on the meter and listen for the effect. From here on tweak accordingly to serve the tempo and feel of the song.
As you bring the attack time down and the release time up, the mix will take on a warmer and soupier character. Slower attack times and faster release times generally will make the mix more “upfront” and “loud”, sometimes with a subtle pleasant distortion character. If you feel that a certain frequency range is still making the mix bus compressor work too hard, you can lightly use a linear multi-band compressor as a “dynamic EQ” to tame the offending frequencies before they hit the mix bus compressor.
Keep in mind that mix bus compression does not have certain rules and it should be viewed as a case by case scenario because every song has a different arrangement, tempo and feel.
Don't be scared to trust your gut instincts when compressing your mix!
Don't push it too far
In the pursuit of sheer loudness, mix bus compression is definitely not your last stop, so make sure not to overdo it. Modern mastering limiters have very advanced algorithms that can take your mix and send it over the edge without destroying all your precious work and making it sound squashed.
It might sound super basic, but if the mix is balanced well, there should be no problems getting it as loud as you want. Beware of wandering too far in the world of loudness though, as more and more online services like Spotify, YouTube and iTunes have started using built in level matching algorithms that can make super loud and squashed mixes sound even more quiet than dynamic mixes that are moderately loud.