This might be stating the obvious but are you able to use your existing or last years/event's attendee data? If you can this gives you a much stronger grounding on which to approach potential sponsors with a proposal.
For example if you can say there will be "x" many senior decision makers from FTSE 100 companies attending, this would be pretty interesting to a b2b technology vendor.
Like @NatashaGiller said, all sponsors really care about is the value to them, and if you can prove this beyond the standard "you'll get brand awareness" then you've got a strong value proposition.
Is this the first time the event is being held?