Best way for mixing a narration with background music?

I can think of one option, reduce the level of the soundtrack so that it doesn't need lowering during periods of narration.
 
What you want is somewhat at odds with

By default the soundtrack should always be at the front

 
You can easily set up an automated ducker and have complete control over how much the level of the soundtrack is attenuated, which I think is your best option
 

How to Duck... :)
https://youtu.be/xaMn3f1dBeE
 
peace!
 

+1 on Jeff's comments. The only thing I'd add is that with projects like commercials and industrial videos, I compress the music's dynamics more than I would normally. This gives a higher average level with a lower peak level, so it still sounds "loud" but the voice can float above it.

Certainly ducking is the way to do it, but it's not always necessary to duck everything. Some instruments don't need to be ducked if their freqency range does not overlap with the dialog. I would arrange the routing so that the instruments that need to be ducked pass through a single bus and then apply ducking to that bus, with fast attack and release. If some instruments are particularly distracting, then I would duck them separately, and less subtly, with slightly longer release time.

I think Jeff said everything that can be said on this topic. So will do nothing more than recap.
1. The best solution is be able to manipulate the original source material so that you can elegantly remove the lead/solo/'singing' parts from the underlying backing tracks during the dialog sections. 
2. Short of that, or possibly in conjunction with it, you can do selective frequency ducking on the source material so as to allow sonic space for the dialog. This ducking can happen either via an actual frequency dependent ducker or via automation envelopes with a good EQ plugin.
3. A combination of 1 and 2 is probably your best bet.

Passive Drift
I thought someone would have mentioned Vocal Rider from Waves as well.
Simplest. It’s on sale right now for 99.00 at the Waves site.

Interesting.
Does anyone know whether Vocal Rider is the same thing as Sidechain / ducking compression, with other name? 
Or better? Or worse?

Anderton
Vocal Rider is about keeping a constant vocal level, not ducking.

This would not be the same as compression?

Compressors were originally designed to automatically ride the volume to even out level fluctuations
 
It was only later they were employed as an effect in their own right as engineers started to add controls like attack/release etc, providing the opportunity to reshape the dynamic envelope.

...wicked
I'm a little confused. Unless you have access to have the music written in a way to naturally leave room for narration vox there really isn't any way to practically do what you're asking without lowering the music volume (via whatever technique works). I'd reconsider your premise that the music always needs to be out front in relation to your narration unless you have a wacky plan for one or the other (doing an experimental treatment on the narration vox for example).
 

The OP is really just asking how to automate the process without having to sit through some long track pushing the volume up and down when each clip of narration start and stops.  
 
The volume changing is the effect he wants, it is the time consuming way of doing it that he is trying to avoid.  Thus why you see responses related to processes that automatically make such changes based on certain variables.  

I don't believe anyway that there is any effective automatic solution.  An automatic solution is always going to sound automatic. eg dialogue comes in, music drops (jumps) down, dialogue goes away music rises (jumps) up to a similar level to dialogue etc. They have automatic solutions on radio but it seems to work there.
 
If you are really serious about this, manual automation is really the only way to go.  What is the problem in doing that.  Are you so pushed for time and the budget so low that proper automation is not possible.  If that were the case I would not even take the job.  Time and money must be allowed for the mixing of the soundtrack.
 
It just sounds so much nicer when this is done properly.  As I have said music can ease down under dialogue and ease or rise so nicely you hardly notice the effect etc.. All the great TV soundtracks you hear have been properly mixed.  Time has been spent in this area.  It is hard work and takes time but the results will show.  You end up with this seamless soundtrack that just flows nicely.  Sometimes it is nice for the music to sit just under the dialogue especially if the music is not too busy at that point, other times it needs to sit a little lower in relation to the dialogue.  These types of decisions cannot be made automatically, only you with your ears can do that.  Same thing when the dialogue stops, sometimes the music might not reach full soundtrack level because it sounds better 3 dB below that. Other times it is nice for the music to ease up to the full soundtrack level.  Sometimes certain vocal words or phrases also need to automated a certain way in order for them to achieve better clarity too.  Something Vocal Rider and things cannot handle that well.
 
Automatic systems and just not smart enough to make those sort of decisions.  I have sat in many soundtrack mixing sessions.  It is like a fine tooth comb over every second of the final soundtrack. That is why it usually takes ten times as long as the program itself in order to do it.  I have seen some soundtrack mixers spend over an hour getting 5 seconds of a final soundtrack mix right.
 
And to your last post too, no, leaving the dialogue at full level and the music to a certain level below that sounds terrible. As soon as the dialogue stops there will be a big hole in the audio eg the soundtrack goes quiet. Not good.  Often a lot of cheap adverts are mixed that way and it sounds terrible.  Listen to high end adverts and listen to how good it all sounds in the end.
 
Listening is the key as well.  Listen to lots of TV soundtracks and you will start to get the idea.