"I think your costume is really great, I would maybe like to chat with you about [thing], maybe we can meet up sometime during the con, here is my cellphone/email address" is a pretty good default.

If the person you're approaching is interested in talking to you, she might offer to meet you at a specific time for a cup of coffee, or will offer to let you attend the next panel with her so you can chat in line... Or maybe she'll smile and nod and say thank you, quietly tucking your business card into her purse and forgetting about it forever, and you're out nothing but a business card. Either way it's pretty painless for everyone involved.


Exception: Do not ask someone to talk about a business thing if all you want is an opportunity to get to know them personally. It is annoying and a waste of everyone's time.

If you want to do business with someone you're keen on, please be completely professional until your business is completed, and then, when they are not relying on you for a product or service (including advice or introductions to important people), you can do the "Hey I think you're neat, coffee?" trick.