I would also advise as a good strategy partnering with an outside group, whether an academic or nonprofit organization, or just an interested academic or activist and their circle of colleagues/students. And make sure they feel this is their event, and give them top billing,
It doesn't have to specifically be women's groups - almost any academic or nonprofit grouping is going to have more gender diversity (and more experience in a lot of needed areas) than what you would catch with just on-wiki outreach.
I would say a measure of success is when you find "new" people are able to "own" this activity in events with different themes, and spreading this type of outreach to more circles and areas of interest, and I think we've seen a fair amount of this in NYC in the past year or so.
Thanks,
Pharos