I generally come up with the idea while we're doing the episode. Last week was the first time where they actually just told me what they wanted while we were recording and I hadn't come up with anything yet. That one's a good one though so I'll see if I can work it into the next episode.
I always go with a nice sharp hobby knife. With sandpaper you run the risk of damaging/over-smoothing out the plastic surrounding the nub. With a blade you can shave off just that part.
Hey, I've been wondering, I just got my first HG gundam model in. What tools do you use to get rid of the nubs? I'm torn between a hobby knife or sandpaper since people say either works.