Don't just pull them off, you will break them. Make sure you've undone all of the screws (underneath aswell), that the ends are away from the arches, and then slide them towards the front of the car. They will be 'stuck' due to all of the mud and shite that's collected between the sills and covers...
You´ll probably have to buy a set of new clips. These plastic parts don´t like to be removed and assembled for more than one time but are not expensive. Begin with the left one for trying - I´ve still got one in store!
just removed mine to waxoil my sills as id crushed the jacking points eek! the white clips do just pull away from the body (towards you) they are slid into slots on back of sill cover but not all the same way so you cant slide the whole thing off , as mentioned earlier remove the screws from underneath first but i found it easyier unclipping from the front section and make your way to the rear one by one i never broke one clip
ive just taken mine off yesterday and the front splitter ready for spraying hopefully should get them sprayed up before sunday ready for stanford hall
i didnt break a clip either! came off quite easy actualy....discovered some of the under seal had come off near the front of each sill and was starting to go rusty so i rubbed it all down and put some more underseal on while ive got the skirts off.
i just hope i dont scrape the splitter when its done and crack all the paint off frown