My advice:
1) Learn what your company's travel/training budget is.
2) Include a professional development component in your work plan if you can. It sounds perfunctory but that can help.
3) Write a paper. That's the single best way to be allowed to attend a conference.
4) Broker a deal. Offer to pay travel expenses if work will pay the registration out of its training budget and give you time off.
5) I agree with the previour comment on writing a trip report and creating a paper trail of progress due to things learned at confeences.
6) Compare for the boss the cost, for example, of YAPC with the cost of a local class.