I use a very small flathead screwdriver
the key to doing this safely and not ripping off more parts of the motherboard is to go SLOWLY and surely. put the screwdiver at the base of the little clips so that it is just under the lip of the clip (it will be parallel to the motherboard and the shield) turn screwdriver about 1/4 to 1/2 turn and the clip will lift. if it doesn't then move to the next one... you can sometimes not get all of the clips to shift, but you can then use the screwdriver directly under the top of the shield and do the 1/4 turn there (just make sure you don't insert tip too far or you risk damage ic under shield)
I start near connector 1 and move down the long side, then do opposite long side, around sim try and then finally near dock. The most difficult part is navigating round connector 4... I usually end up using the screw driver under the shield instead of trying to lift tabs.
once you've lifted the outer edges of the shield you'll be able to either bring the 2 halves up enough to see the centre join and slip screw driver along to lift the tabs/shield. As long as you are careful you can just pop the shield back on again-the foil on the join will be wrinkled but who cares?