The big picture
When you hear a symphony orchestra playing Mozart or Beethoven, or a brass band pumping out a Sousa march, you’re hearing first hand how a well-crafted arrangement creates a good-sounding live mix — without the aid of a PA system and front-of-house engineer. In these musical styles, as well as others that were born before the days of multitrack recording, the total sound of the ensemble was thought through from the beginning. Today that's often not the case, as many parts are recorded as overdubs, and it's often left to the mixer to make the final audio presentation coherent. Applying some of the old-school mentality to your arrangements can only help, as it will force you to analyze what’s going, and should lead to more "studio-ready" arrangements.
Let's start by thinking about frequency. Think about how tricky it often is to create sonic space for each instrument when you're mixing. When instruments occupy a similar frequency range (especially in the low end and lower mids), EQ or panning is often required to avoid a cluttered, muddy sound. Here’s an example. Say you have a guitar and a piano in your song’s instrumentation. If your arrangement has them both playing chords in the same octave, they may very well get in each other's way, and you'll have to put some effort into giving each its own space.
But a simple change of register for one of the instruments should open things up quite a bit. By moving the piano or the guitar up an octave, for example, there won't be as much sonic overlap, and thus less clutter. Those are the kinds of things you should be thinking about when arranging a piece of music for the studio, or for live performance. If you're able to fit your instruments into their own space, frequency wise, there will be less headaches on the mixing side when you're in the studio.
It's not just frequency, though, it's also the arrangement of the parts. Say, for example, there’s a section in a song where the drummer switches from the snare to the floor tom. If the bass guitar is playing a busy part at that point, it probably will compete sonically with the floor tom, as they’re in a similar register. So that might be the spot where you ask the bass player to simplify his or her part, so that the floor tom can pop out more.
Now let’s look at dynamics. If you change the energy level in different parts of the song, you can create dramatic impact. In written music, there are dynamic marks, which tell players how loud or soft to play in various parts of the song. But especially in a band context, the written music, if there's any at all, is often just a basic chord chart, without any dynamics marks. So if you’re arranging a song for your band, it’s incumbent on you to figure out where you want dynamic changes to occur, and to make sure to let the band know. A song with dynamic changes will sound better live, and in the studio.
A song recorded with dynamics will also be easier to mix, because the contrast in volume levels creates more drama and excitement. You won’t have to try to artificially create dynamics by bringing up, say, the drums at a certain point in the mix — they’ll already be there!
Also important is the actual song form, where a well-crafted bridge can add contrast and a change in feel, and setup a more dramatic ending. If you're not involved with the songwriting, however, you can still add additional contrast by putting in a breakdown section. Whereas a bridge utilizes harmonic (chordal) changes and possibly a change in feel to provide a change of pace, a breakdown section does it dynamically. Breakdowns are used a lot in dance music, which would otherwise be dynamically static. In a breakdown, instruments drop out or play simpler parts, and the reduction in energy sets up the next song section (typically the last chorus or last verse and chorus), which will then seem more powerful in comparison.
The takeaway from all this is that if you craft your arrangements intelligently, and take into the account the factors mentioned here, you're likely to have a much easier time when you get to the mixing stage.