Hah! I like Achilles and MH's answers.
For serious answers, which is what I suspect you would like to have after all of the humor, I think that Achille's first answer is good. It has the additional benefit of disabling use of their hands for other purposes. Beyond that, you can also have them hold the skirt up while keeping their hands in front, by just holding on to the sides of the skirt and pulling some tension on it (especially useful if someone with limited mobility or shoulder problems cannot reach around behind).
There is the mentioned waistband tuck, of course, if there is a waistband to be had. It can also be tucked into a belt, if there is one present (not common, but occasionally used in various fashion schemes). Tops who like corner time (I'm not a particular fan myself) can keep their own supply of safety pins handy. Larger is much better, because depending on the type and style of clothing worn, small ones may not be big enough to go through the material. The last thing you want to do as a Top is to run the pin through your finger trying to work it through their clothing and give a big "Youcchh!," resulting in much satisfaction for the just spanked girl.
