Mention things you have in common, and maybe ask a question or two about them. You are trying to get a conversation going, so asking questions is a good idea, as long as you don't ask too many. Also try to avoid long letters with little details that might be considered boring. Keep it to the point, but try to let your personality shine through. One of you may be new to the area, and could have a lot to talk about it.
Music is also a good conversation starter, most everyone likes some kind of music.
Hopefully, the person's personal ad showed you the types of music they like, so you know where to go with the conversation. Whatever you do, don't talk about the weather, unless it is really extreme. It can be a very boring subject. You are more likely to get a response if you are more light-hearted in your first contact. We hope these tips for replying to personals are helpful.