It sounds like you are talking about stretching the labia minora (inner lips). Wikipedia does cite some texts that document African tribes who traditionally elongated the inner labia but I don't know exactly how they went about doing this. It's interesting that in some cultures, women try to make the inner labia protrude more, and yet in other cultures (for example, the U. S.) many women undergo operations that reduce the inner labia.
This page might help with some more info on stretching and training:
(inner) http://www.sexylabia.com/labia-enlargement1.htm
Among other tips, this site will advise that you do not always pull the labia from the same spot, but alternate between pulling at the front, back, and middle so that they develop evenly.
I have also read about vacuum pumps with attachments specially designed to puff up the labia majora (outer lips), and supposedly women who use this device often enough will get their outer pussy lips to maintain a somewhat puffy, swollen appearance. You can read about this and take a look at the device at the following links:
http://www.wartenbergwheel.com/Camel_Toe.html
http://www.wartenbergwheel.com/cupping-enlargement.html
(outer)- http://www.sexylabia.com/labia-enlargement2.htm
The entire "sexy labia" site is actually pretty informative and I would recommend that you and your girlfriend read every page of it, if you are both seriously interested in safely and effectively indulging in the practice of labia training: http://www.sexylabia.com The table of contents provides links to quite a few articles that offer a great deal of info about this, and when you click on the photos a lot of them have captions that contain tips and advice as well: http://www.sexylabia.com/table_of_contents.htm . In fact, the site even has links that offer some guidelines on proper pubic shaving without irritation, since smooth skin is necessary for a vulva pump device to form a seal and create suction.