April,

Your work is yours until you transfer ownership to your client. If her site, flyers or whatever use your copy but have a (C) symbol in her name, then you cant do anything about it.

As Alex said, put it in writing and I would even say all work becomes the property of the client only after payment has been made according to agreed upon terms. Then list those terms in writing.

That way, it keeps her honest in case of non-payment.

Good luck.