Following yesterday's advice on responding to reviewers, today I saw someone on academia.edu post this rough guide to conference speaking for graduate students.
Generally, I think it's all pretty sensible, with perhaps a couple of caveats. He doesn't think much of graduate conferences. I agree they're not much of a CV boost. Moreover, at somewhere like Oxford you can probably get better feedback from presenting in some of the internal seminars on offer. I think they are worth going to just to meet fellow students, who could be future colleagues, though.
Also, this one: Apply to conferences even if you don’t intend to go. Being invited to speak is still worth having on your CV, even if you ultimately don’t attend. Declining an invitation due to ‘unforeseen circumstances’ will not cause offence. Of course, don’t do it too often, you might get a reputation! rankles with me a bit. It just sounds cynical and I'm not really sure that putting an accepted but declined presentation on your CV is any real boost either.