I've recently had to take both my side skirts off to clean the jacking points so I took some pics of how I did it.
A bit of lubrication never hurt anyone so first off I sprayed some WD40 over all the clips (expanding plastic rivets); there are two at the rear of the side skirt on the arch lip and 6 or 7 under the skirt.
I also used the little the tube to spray some on the top clips; you can pry the rubber gasket along the top of the skirt back just enough to squeeze the tube in and lube up the white clips.
The clips on the rear arch lip look like this:
The centre piece pushes through the middle and then the sleeve can be pulled out. If you're lucky you can push the centre piece through just enough to release pressure but not enough to lose it. In reality it falls into the sill though
The clips under the skirt are similar but you can actually pull out the centre piece.
Now everything is loose it's time to un-clip the skirt. I couldn't have done it without this tool (and I didn't break any clips either :jaw-dropping )
The way I found worked best was to pull the base of the skirt away from the body slightly and then 'pop' the top off the clips with the tool slid inbetween:
Once off it should look like this:
I took both off and re-fitted them without any problem using this method; I may have been lucky not breaking any clips but I think the WD40 helped a lot.
Hope that helps