For a quick and dirty (pardon the pun!) bobbin case cleanup, I use a small makeup brush. A narrow-tip brush fits well into the crevices of the bobbin case as well as the sides and the back of the bobbin area. A wide-tip one (like for spreading blush) works really well for dusting the bobbin door.
I bought an inexpensive set of brushes specifically for the purpose. (The brushes should be good enough quality to not lose bristles inside the machine, though.)
This quick cleanup isn’t good enough to replace a thorough cleaning, of course, but it seems to tide my machine over quite well. Besides, it’s so fast and easy that I’m more likely to do it often. 🙂
Do you have any favorite tips for sewing machine care? Do share!