As your work develops, it will be inclined to sprawl - to seek the broad shallows when in fact you need depth. To corral it and contain it so that you end up with something pointed, something that makes a contribution rather than simply rehearsing what is already known, you need a device.
One option is to write a sentence that tells you precisely what your thesis is all about. This is remarkably useful when people ask you what you are researching. Be very strict with yourself: keep it on one (reasonable length) sentence. This forces you to pay attention only to the important features of the work; and reminds you where the red herrings lie.
For example: 'Life writing is a genre that brings with it well known problems to do with copyright, and the ethics of representation, but the psychological effects of writing a 'true' life is a problem that has been under-researched; this thesis explores the effects on authors of undertaking a life writing project'; or 'This thesis investigates poetry as a mode of knowledge, and seeks to identify how poetics, aesthetics, form and observation together comprise a research methodology'.
By writing and rewriting such sentences over the course of your candidature, you will be able to refine your topic, and keep your eye on the main point.
For those who work well within tight confines, and who enjoy language and play, try doing the same thing but in the form of a haiku, rather than a sentence. This gives you only 17 syllables to play with, so it is not easy; but it relies heavily on metaphor and movement, so it is very well designed for creative writing theses. (Note: the quality of the haiku is not at stake; what is necessary is the capacity to find the heart of your topic.)
An example from a poetry student:
Time evades each pen
Is memory a poem,
or can poetry frame a life?