Since I lost about 7 pounds, my jeans have looked too baggy and have
fallen off my waist. Tightly-cinched belts don't help because the
jeans's fabric below the waist is still too loose. I don't want to pay
to buy new jeans because my weight often fluctuates. I have tried
using pins to alter the fit, but they are obvious and fall out much
too easily. Is there any special type of pin or tool designed to
customize the fit of clothing? Thank you!

You could try adding elastic to the back waistband.

Kriswrite

Get a sewing machine or someone to take in a tuck along the back seam.
It could be removed if you expand again.

Safety pins?

Try sewing the side seams (around the hips/upper thigh) a little bit
on each side. Leave the original seams then you can unpick the new
seams if needed. Use Safety Pins to test the fit before sewing.

Good Luck :)