I'm currently writing a new novel, and have been working hard at pushing myself both to write faster than usual, and (my perpetual goal) to make it the best thing I've written so far.
I've been thinking a lot about tone lately because when I'm actively writing a novel, I try to read (ie: listen to) really well-written novels. I either re-read specific books or choose new books from critics 'best of' lists, award-winners from previous years, favorite authors, etc., to help inspire me to strive for the same level of skill in my own writing. I also tend to pick novels that have a similar tone to what I'm writing, to help keep me on track - strange and unpredictable (current WIP), lyrical, dark, complex and political, noir, and so on. As I listen to these great novels, I take note of the things that most impress me, whether it's Gregory Maguire's beautiful use of language, Neil Gaiman's ability to always take me somewhere I wasn't expecting, or George R. R. Martin's depth of characterization.
It's strange but, for me, the tone, even once I define it, remains a fairly abstract thing - a sense of what I want my book to be - but it very much guides my planning and, later, my writing. It's the same as if someone asked me to define what my life has been like; I doubt I could put it into adequate words, yet I have a 'feeling' that I associate with the experience as a whole. In the same way, I 'feel' my protagonist's life and emotions, and that, in turn, guides how I write their story. For example, I already have a concept for a new project, but all I know about it so far is that it would be a contemporary fantasy, gritty and noir, and that the films Sin City, Memento and Constantine keep coming to mind. Not sure where it's going to go from there but there'll be time to think about that later. For now, it's back to work on the current WIP.