When applied to glass, products like these create an invisible barrier that causes water, oils, and debris (like soap suds) to bead and roll off.
Find this product anywhere that sells basic auto supplies. You’ll know it’s time to reapply when water stops beading on shower doors. Keep in mind, windshield rain repellents were made to treat glass, not plastic.
Also, wiping down the glass with a towel or squidgy after showering with keep the water and soap off.