Subject: How do I stop moisture spreading bacteria on agar plates stored upside down

I am culturing acidophilus in petri dishes containing 
nutrient agar and, even though the plates are incubated upside 
down, moisture still condenses on the surface of the agar in 
large drops.  These drops move across the surface, spreading the 
cultures everywhere.  Since I am growing cultures in each of the 
four quadrants of the dish and want to keep them separate, the 
moisture is causing problems.  What am I doing wrong? 

Re: How do I stop moisture spreading bacteria on agar plates stored upside down