For most of the descent phase in the C172, normal (or just slightly reduced) cruise power works a charm. It's not exactly a rocket, you know, and the added speed is usually welcome. For the final descent less power is obviously required, and the other tips here about avoiding shock cooling could be useful.
"Clearing the engine" my momentarily adding power and reducing it again, to clear or prevent spark plug fouling has always worked very well for me, in both 152s, 172s and 172RGs. Normally there would not be any fouling to clear, and the few times it was (long idle descent), it would clear within a few seconds, leaving the engine running just like a song.