Hi,
I use a program called CutePDF writer. It is a freeware. You install it as a printer and use it to print directly into a PDF file instead of a printer. You can then print this PDF file on any printer of your choice as any other PDF file.
On the advance section of the properties, under Postcript, you have the choice of making a mirror image. I just tested it and it work very well.
If you use any PDF file to print your desing, for toner transfer for instance, be sure not to leave "adjust to the zone of impression" in the scaling option. Select "No scaling". The wording may be a bit different, my copy is in french and I just translate from my copy at my best.
Alain