Honestly, I have found the simplest methods to be most effective (in terms of cost and quality).
In effecting drawing duplication, I have used two methods, both of which are fine for stage shows.
1. Have a book readily available for the participant to use as a hard surface on which to draw (e.g. have it sitting within reach after a book test, or find some other logical reason that it should be present). This approach works best for more impromptu settings that happen to involve large audiences.
2. Use a clipboard and the oldest method around (CP). All that is left after preparing the clipboard is to justify glimpsing the drawing, which can be done in any number of ways. For example, hand her the board and a pen, have her draw her image, tell her to remove the sheet and hand you the clipboard. Realign the pages, as they most likely became disheveled when she took her sheet off. You are turned with your back toward the audience as she folds her paper, so you have plenty of time to find the sheet with the i********n, glimpse it, and proceed (all in the process of straightening up the pages). In actual fact this doesn't really need to be mentioned. While your back is turned, she is busy trying to keep you from seeing what she is doing, and thus is probably very involved in her sheet of paper as she folds it and so forth. You can just quietly glimpse the i********n while she is busy and the audience watches her. Should you need to stall for time, offer her a dark marker as an afterthought, saying that she should probably make the image more clear for everyone to see (assuming she was using a ball point pen previously). Of course, it makes sense to have some music playing as she does this so as to make it less of an awkward moment.
Anyhow, these are the methods with which I have experimented, and they seem to have worked rather well. Furthermore, they cost about $15 per year, if even that much. If you ask me, that is more than most mentalists could ask for these days. I know about the complaints involved with using more old-fashioned methods (i.e. book surfaces and CP) but these drawbacks can be effectively overcome with enough experimentation. I feel that most people simply don't make the effort to identify and eradicate the weaknesses.