I haven't fitted my VW/Votex skirts yet, but they come with an Elch Technik P1-Adhesive Kit made by Rhodia Silicon GmbH. Its a 1 component polyurethane adhesive, but its German made so the skirts will probably never come off again!
Its probably mega hard to get hold of so I'd look at finding your nearest Wurth dealer and hunt down an adhesive from them. They're another German brand but they sell their automotive products to garages and bodyshops worldwide.
EDIT: Just looked on their site and they also sell 1 component polyurethane adhesive but only recommend using it on wood! Now I'm going to look for a new adhesive, I'll let you know what I find!