if you have a 60 year old tub...clean it regularly.
folks, i can't stress this one enough. mr mfranti and i are active people. that's a fancy way to say we get really dirty. especially our feet. all that good dirt and grime that accumulates after 10 hours in chaco sandals gets transferred to our overly porous tub where it sits shower after shower, layer upon layer, month after month.
to conquer this, i spent an inordinate amount of time with dish soap, laundry detergent, baking soda and some super fine steel wool scrubbing until my back couldn't bend over anymore.
to avoid this problem, keep a scrubby dude in your shower. during your regular bathing routine, like when you are lathering up or conditioning, use your foot to scrub off the dirt. do this regularly and you wont have to suffer the way i did.
also, keep a squeegee in your shower to wipe off the water after your shower. trust me, you will save yourself a lot of grief if you do this daily.