When I feel one coming on I know if I catch it early enough it won't get to 100% if I can get to sleep in time.
I take three Aspirin
I lie on my back in the bathtub with my face under the shower - the sensation of all the water confuses the pain signal and I can't actually feel the pain behind my eye while I'm under the shower
I have to be lying down so the water is going at enough velocity that it makes my face numb
I stay there for as long as it takes for me to feel like the drugs have started to kick in - usually half an hour at least
I get up and go straight to bed and usually fall asleep within ten minutes
When I wake up, the pain's usually gone.
For me, this works every time.
If I don't catch it in time, I do pretty much the same thing, but go to bed after I vomit.
I've worked out my triggers are smoking (back when I used to), and significant disruption to sleep (i.e. working all night) or significant disruption to meal patterns. Giving up smoking was the best thing for me in terms of reducing incidence of cluster headaches.