I always change my strings all at once; while they're off, I'll first take one of thos four-sided foam manicure sanding sponges and shine up the frets. This is much easier on the steel than files, and the two finest grits will put a fine shine back on them. (These also are cheap and handy for the brass parts as well.) I then take lemon oil (I prefer better oil from a woodworking shop), apply it liberally and let it sit and soak it up for about 20 minutes.
Wipe off what's left after that, string your bass back up and you're back in business.
I'll repeat my other cleaning tricks:
There's no better lubricant for nut screws, bridge screws, etc., than BreakFree CLP. Usually found where firearms and accessories are sold, it's finish and electronically inert, and is sweat proof. Standard issue with Uncle Sam from Berettas to Howitzers.
For gold or plated parts, a great preservative is VO5, the hairdressing in the gpld toothpaste tube. A T-H-I-N film will keep gold or chrome shiny for a long time. All organic and harmless to the instrument.
And for those of us with the glossy finishes, nothing works like MEGUIAR's or MOTHER's cleaners and waxes. If they're good enough for $100K paint jobs on show cars at the Oakland Show, they work great on the best guitar paint jobs from ALEMBIC.
PS: Wax is great for getting that cloudy look off of shiny parts, like the tuning key buttons.
When I go play, I like mine to be like a show car when I pull it out of the case. Everything is perfect, and the prettiest eye-candy most people have ever seen. Then the sound seals the deal!
J o e y