There is one sure fire way I know of that means I can get 3,000-words or more a day, and that, it variety.
(Love this from marriedtothesea.com - it speaks to my writer's soul)
I know there are people out there who work on one book at a time. That's awesome. I cannot physically do it. If I get to a place where the words refuse to flow, I need to have someting to fall back on.
Yesterday, I worked on 4 different manuscripts.
1) The next Space Western in my Flynn Monroe series
2) A YA paranormal that's just for fun
3) A SF where I pit a protagonist and an anti-hero against someone who's still called "dudeguy" in the draft.
4) A Victorian era book of steampunk and intrigue.
If I get mired down in something, I hop over to something else and try to work out my issues by falling into the flow of another story.
It usually works. Sometimes it doesn't but most of the time, it does. When I have to fight through a scene my wpm count drops drastically.
So, if I get to a wonky bit, I pull ot something else dust it off and get to work on it instead It does mean that I have a lot of things in the 30-80% done range right now... but I'm okay with that.