Contrary to fondue's statement*, I am under the conviction that "wanking" is synonymous with masturbation - gender irrelevant. Female friends of mine often refer to their daily rituals (yes, daily) as "wanking", not frigging.
Even if I am wrong, let's just assume that "wanking" in this node means "masturbation". In this case, female masturbation.
How to masturbate discretely as a female
Before I get started, I should probably point out that I am a guy and thusly can't vouch for these personally - however, they are based on hints and stories given to me by female friends. So they might work. Or not. If it doesn't work, at least you'll only look silly once.
The most popular method of masturbating discretely for girls/women is to simply rub their legs together. If done at the right angle, it can provide mild stimulation to the clitoris. This is particularly useful in public or in crowded places - as long as no one is watching you directly (your facial expression will probably give it away). However, it does come at the cost opportunity of having to go to the loo to finish the job.
Besides, in any case, a woman masturbating is far less repulsive than a man masturbating. The "standard" mental image of a man masturbating is a sweaty dork with his pants around his ankles, ogling at naked chicks on his computer screen, and beating his monkey like a possessed man. The "standard" mental image of a woman masturbating is a sensual, enduring process which starts at the breasts, continues onward to the vagina, and gradually builds up to a clinching climax of ecstasy - all while soft jazz music is inexplicably playing in the background, and dimmed lights.
Of course, reality is far worse, but stereotypes wouldn't have it any other way.
* Since removed from this wu, presumably by a deity of some sort.