On the subject of Hughes' letters, Olwyn Hughes has just sold a bunch more of Hughes' letters to the British library.
The letters apparently include add-on's by Plath and provide further insight into both of their respective creative developments!
See here for the British Library statement and thanks to Peter at Sylvia Plath Info blog for the heads up.

Advice from Ted

I've spent the morning immersed in the Letters of Ted Hughes trying to soak up any writing advice.

"If you write whatever attracts you, and you write it as hard as you can, and as rich, then you can't miss...[j]ust write it off, in your own way, and make it stand up off the page and jump about the room" (p66)

"Insofar as poetry is convincingly the real speech of a real person, it seems to happen "off" the page. I think that's fairly true" (p373)

"poetry is simply the name we give to a certain kind of writing...inner concentration, inner listening, dependance on the spontaneous mind rather than on cultivating & remembering mind... So rather than saying 'study writing', I was saying 'practice writing', as diving to depths has to be practised.
The whole business closer to athletics than to aesthetics, perhaps. hence my no words about 'technique'" (p483)

"My definition of 'poetry', almost, excludes anything coming from the ego under the ego's control" (p628)

"The deadliest thing is for a writer to develop too fixed a 'style'. The ultimate, to my mind, must be the naked voice of that inner being" (p636)

 "A feeling is always looking for a metaphor of itself in which it can reveal itself unrecognised" (p678)

I'll add more as I finish the book, but these quotes speak immensely to me, especially the 'practice writing' as opposed to 'study writing'. I think too much about how to write and simply don't 'practice' enough, fairly obvious advice really...