Rub hard soap or paraffin wax along the zipper.

Keep the tension off. Don't latch the top until it is zipped.

My advice: leave it zipped, and use a rolled towel to keep the window
from creasing when you drop the top.

My zipper lost some teeth four years ago, and after getting it zipped
one last time (with great difficulty) I decided to see how much longer
the original top would last without unzipping. The window is a little
wavy now, but nowhere near bad enough to worry about. My top goes up and
down frequently, though it spends most of its time down. 11 years and
counting...

---
Lanny Chambers
'94C, St. Louis
http://www.hummingbirds.net/alignment.html

A few years ago I read (on miata.net?) about a "Zipper Rescue Kit."  See
http://www.zipperrescue.com/special.asp  Basically for under $15, you get a
hand full of new sliders (the pull part), including a 9mm slider used on
Miata zippers.  It worked pretty well for me.  But alas, even that began to
fail and I resorted to the put-a-towel-under-the-window trick.  Actually, I
used a "noodle" more commonly found at swimming pools.

Tom
93C

An occasional swipe with a black crayola along the zipper will do
wonders and it doesn't leave the little white pieces that parafin wax
does.  It may not work well with yours until you get the zipper
working well, though.