That's a very good question. One that I'm surprised doesn't get asked more often!

Back in the day, it was pretty much all index finger work.
Then I found Tadpoles!

NOOO, not the recumbent trikes. Although I have found them two

Tadpoles are little rubber sanding grommets that are made for detailing woodwork.
I tear a sheet of sandpaper into 20 pieces and wrap the pieces around the tadpole and have at it.
They come with many shapes & edges. The last set I got was 12 tadpoles for like $18.
If I had my old computer, I would have the link handy, but I know that I got them through my friends at
shop.com

But as cool as Tadpoles are, I still have many other instruments for those BB/chainstay areas.
One is an orange stick. Used to push back cuticles. I've sanded the ends to get into really tricky spots.
I have also shaped paint stiring sticks and other hardwood into many sanding tools. Even a standard pencil with eraser.

Also be sure to have the BIG tub of elbow grease handy!