Does anyone have any secret tips for cleaning fir needles out of a cactus that's been outside all summer?

I use scissors for that kind of cleaning. Use the scissors like tweezers, with just enough pressure to grab but not cut. If you have any old, dull scissors, this could be their moment of new glory.

If it wasn't a plastic pot, you could turn it upside-down but roots in plastic usually slide right out.

Another option could be to use a chopstick or shish-ka-bob stick with something sticky on the end, like gum or tape.

When it's time to water, I rinse the plant with a strong stream of water, most of the debris will wash away. Anything that's left, I carefully pick out with hemostats. If anything remains after that, it will just have to biodegrade itself, don't worry about it.