My technique was always to take an unbent paperclip or other rigid wire, scrape the tip of it against the middle of the screen (of course, not so hard as to damage the screen). It made the screen vibrate enough to knock the keif off it.
I used to just hold a lighter 6 inches below my grinder screen and basically vape whatever was clogged in the screen. It always worked really well.
I used to have a grinder with 2 screens in it and the top one would fill up reasonably fast and the bottom one took a while, but the kief out of the bottom was fucking incredibly.
