Fill your bathroom sink with about half an inch to an inch (around 2 cm) of water. Put the top of your cup against the side of the sink. Move the cup down so that the top is under the water, against the bottom of the sink. Squeeze the cup. When you squeeze the cup, air bubbles will be forced out of the holes, and that will remove anything that's clogging them.
I use a small Diva, so I don't know if it'll work the same for the other cups, but for me this technique works better than the standard one.