<< My wiper blades periodically need tightening,and rather than getting out
 and re-tightening them at inopportune moments, I've been using Rain-X  >>
Try this:
Pull the arms off and with a needle, clean the grooves on the conical part of
the shaft that the wiper arm fits on.  Then re install the arm. If you want
to get all crazy, just replace the nut with a nylon locknut or do a tiny dab
of blue loctite.  The soft aluminium strips out of the arm and becomes lodged
in the shaft's grooves.  You then run the wipers and the drag of the wiper on
the w-shield works the nuts loose.
Btw....RainX is a joy, especially on that big 'ole flat windshield.
Patrick