Before I start editing a trailer, I go through each music cue I'm going to use, and add colored markers to "map" out the shape of it. This lets me know what is going on during each section at a glance, which saves me a TON of time.
If there are no markers of any kind on the music you're editing, then every single time you need to find a particular moment, you have to scrub around, and listen to find the section that you're going to edit.
Markers of any kind are a step in the right direction, but if they're all the same color this still only tells you where the beats are, and not what's going on in that particular section of the cue.
Premiere Pro, Avid Media Composer and Final Cut Pro 7 all support different colored markers; as of the writing of this post, Final Cut Pro X does not yet support multiple marker colors.
So without further ado, here's my system for preparing music for editing!
Green markers - Indicate when a cue repeats an idea or explores an idea without significantly changing the intensity. A cue will often repeat an idea once or twice, especially in production music made for specifically trailers. These sections are good for when the energy of the trailer needs to stay roughly the same, for example when a character is giving an expositional speech. In that scenario the music needs to be present, and ever so slightly ramping up the energy, but it's not at the forefront of the trailer.
Red markers - Indicate when the melody of the music changes. These moments are for when the emotion or action of the trailer needs to change or escalate. The red markers allow me to quickly audition each "new" section of the cue to find one that suits my needs. I define a “new” section as one where either the melody changes; the tempo increases or decreases; the instrumentation gets more complex, etc.
Cyan markers - Indicate a "fill" which Wikipedia defines as:
"a short musical passage, riff, or rhythmic sound which helps to sustain the listener's attention during a break between the phrases of a melody."
These moments are good for cutting in short bursts of action in between dialogue. To take a very typical example: if a character says "This woman is the best in the business!" that might be followed by three shots showing how amazing she is at her job, before cutting back to another line of dialogue.
A fill is a great musical way to emphasize those cuts. Even better, they can frequently be taken completely out of their original context, and used where necessary. This works especially well when the melody drops out, and a bunch of heavy drumbeats play.
Another way I use a fill, is to transition between two different moments in a cue that otherwise occur at two completely different sections. Sometimes I also just use cyan markers in a cue to mark a short one-off musical moment in the middle of a section.
Yellow markers - Indicate a "rise" in the music, usually before a really big beat or hit. These moments give a sense of anticipation to the audience, and are a good way to lead up to a music stop down or a climax.
They often take the form of a drum beat that rapidly increases in tempo, or maybe some increasingly urgent strings. Often I'll augment these moments with a sound effect rise.
Purple markers - Indicate really big beats, big changes in the music, and sometimes a stopdown in the middle or end of the cue. These beats will usually go with the most dramatic scenes or biggest action scenes in a trailer, and of course the epic finale before the title.
After I've added these markers, I now have a visual map of the cue, and I'm prepared to edit.
Being extremely organized is what allows you to edit quickly and efficiently. Nothing slows the creative process like repeatedly searching through a music cue to find what you need, so spend the time up front to save it down the line.
You don't have to adhere to my system; maybe you'll develop your own that suits the way you think about music. The important part is just having a way to know precisely where to go when you're looking for a particular musical feel.
Also watch my video on how to make music reverb out, which is another great tool to have under your belt when the music doesn't end when you'd like it to.
Good music editing takes a while to learn; it's the first thing I look at when assessing how much experience an editor has. Great music editing will shape the pacing, highs and lows, and tone of your trailer.
And remember, if you just can't seem to get a music transition to work, just put a whoosh sound effect under it :P