Something about the girl's ad, something about you, something funny, something about seeing where things could lead.
First, write that you read her ad and liked it, she sounds like a girl who is smart and nice/knows what she wants/is looking for the same as you. Also, you like it that she (something particular from her ad here, to show that you are not just copying and pasting the same message to dozens of women. Of course you are copying and pasting most of it, why wouldn't you? But the parts about her, those parts should really be about her, not just a compliment you use for all the girls.).
New paragraph: "I am a...." you describe what you do, and how long you have been single. (It is interesting for her to hear if you just broke up last week, or six months ago, or five years ago.) You write what you are looking for at this point in life, and since you know two guys who met their girlfriends through the internet, it seems like it's worth a try. (By mentioning that, you show her that you're not just online because you are chained to your computer. Also, you show her that she doesn't have to feel embarrassed for using internet dating, that you are treating it as a normal thing.)
Try to insert something humorous, mildly humorous that is, throughout the message. If you can manage to write something humorous it proves your intelligence, and that you aren't desperate; you show that you are on top of things. Keep in mind, this girl wants to be certain you are not a desperate loser, so you need to show that you are a relaxed guy who is writing without too much emotion invested in it. You have a smile on your face and a devil-may-care attitude. Which isn't true, of course you have written to a lot of girls already and it's frustrating as hell sometimes, but she doesn't need to hear that.
A suitable joke could be when you describe what you are looking for: "Fluffy hair is a must, inherited fortune is optional." Or: "However, none of this matters if you know how to make ravioli. Mmm, ravioli...."
MAKE SURE that you add something active, somewhere. You are not just a couch potato. You write that you were off camping last weekend, or went on a road trip with your buddies last month, or you are cutting the email short because karate class is starting in an hour and you don't want to be late. Make sure you give her an activity to picture you in.
