Originally Posted by JohnMc
Budzu, when I pour plates in the lab...
...I have been known to place a bottle (or something that completely covers the diameter of the plate) full of very hot H20 on the top of the stack of petri dishes until the agar solidifies. This eliminates condensation in those top 2 or 4 plates. No condensation should be present in the middle plates, at least in my experience.
Storing poured plates cold encased in plastic will lead to moisture most of the time. Try parafilm instead, or keep at room temp. RT will allow any contaminants to make their presence known before you streak on them.