As far as kick / bass ducking I can't help. I hate the very idea of it.
Sidechains are infinitely useful though.
Mult the signal out to EQ back into the sidechain to squash down nasty low or high frequencies extra hard.
Mult stereo signals to mono, route to a pair of mono comp side chains for stereo compression without left-to-right shifting. Try it on hard-panned overheads, for example.
You can use it to fatten up tone. Someone recently posted a great tip for kick drum: feed a sine wave into a gate. Send the "real" kick drum into the side chain. Mix the output of the gate in with the original "real" kick. The sine wave should be around 150 Hz for a huge kick, 250 for a jazzy kick. Fast attck, fast release. (Apparently longer release works for a hip hoppy sound, but I wouldn't know). I can't remember who posted the idea or what thread it was, but to somone: thanks! Great tip!
I'm sure there are loooots of other uses for sidechain...
Have fun with it!
Cheers,
Johann