Automatic Ducking

I use duckers quite frequently when mixing TV spots & content, and I think I have a pretty good formula. You can use any compressor with a low enough ratio (1.25:1 or lower is acceptable). I use the R-Compressor from Waves at between 1.10:1 and 1.13:1 with a very low threshold (-30dB). This causes about the same amount of gain reduction regardless of how much signal is present in the VO. (when there's voice, there's gain reduction in the ducker). Your gain reduction at any given moment is less than 3dB, but it really helps the voice pop. I use very fast attack and release times for this type of ducking.

I think the secret to making it work well is to not engage the ducker until you've automated the volume on your VO (and most likely your music, too), EQ'd the VO and music, and applied any other compression that you'll be using.

As for EQ, I'm usually pretty gentle with music tracks - but that depends on the music. If you have strong horns, strings, guitar, piano, or vocals in the music, the EQ can be essential. Otherwise, for general "make room for the VO" EQ, I'll use subtractive EQ in the upper mids - maybe -3dB, wide Q in the 3k or 4k range - but it varies. I'm also a big fan of narrow additive EQ to bring out a bass element or something that adds character to the music and the spot without interfering with the VO (when it's called for).

Keep in mind that any compression you do on the master (the use of limiting such as L2 is another topic altogether) will act similar to a ducker in the sense that if your VO is mixed hotter than the music, everything gets compressed when the VO hits. Especially effective in this area is a multiband compressor like the C4 or Linear MB from Waves, which will cause mid-range or low-mid gain reduction to your mix during VO lines. Just BE CAREFUL. Don't let any compressor or limiter work too hard without being able to justify it.

After all this talk about compression, remember that you really want to get your spot leveled out before adding any duckers or master compression. Volume automation is your best friend for a great spot.