So, I know that I am going to turn an offer down but I'm genuinely concerned as the right etiquette for doing so. As with all of the places I applied to, I built up a rapport with a prospective supervisor who was kind enough to read and comment on SOP drafts and even my writing sample. She's a major player in my field and I don't want her to think I have been ungrateful for her help and support, however, I got into a school that I really didn't think I would do (with a much better finance package) and so I do need to turn the other offer down.
How do you go about doing this in a way that doesn't sever ties (as I say, in my field I will certainly encounter her again) but does acknowledge that I've given the offer serious consideration?
(apologies of the jackass topic title - I have no intention of writing that in the email!)