You know how you have the forums set up where you can see the last x number of comments when you reply? Can you do that in the Private Messages? Just for the specific one you are replying to. I tend to get on a ramble and space out what the point I was making really applied to. Be cool to look below and make sure I was covering everything the person who wrote me was talking about without opening a new window to see the message I was replying too.
See how I ramble?