Working on multiple pages at a time can keep you from slowing down or getting bored. I may just be in the mood to draw faces or just do layouts, so I choose the pages I'd like to work on, then when I stop feeling picky I can go back and finish the upcoming pages.
Thumbnails don't have to be that big, As long as you remember what you want, they can be squigglies the size of a quarter. This way you can fit 30-40 on a page. I usually look at the dialogue document while I'm doing this and shape the panels around the character's poses.
I do have slow weeks though when I feel unmotivated or have been stuck on a scene for too long.
Also developing a style/method you know you can work fast with is another thing. Took me a year to find a style I could produce fast with and was satisfied with the quality. I don't do full digital, so I could also just work faster with traditional materials instead of always undoing lines and drawing until the line's right. Once it's inked there's no going back.