Water in my gas tank.

So, I posted on here a while ago with a problem i was having.. when i would accelerate and get up to speed my motor would almost cut out, then back in,then out until it died... i changed the fuel filter and messed with some other stuff.. well i decided to take my fuel filter back off and empty it into a bowl to check for water in it.. and sure enough there was.. which means i have water in my fuel tank. Im going to siphon out a few gallons from the bottom to get it out. my question to you guys is, how do i keep this from happening.. and how did it happen when i put a fresh tank of gas in there? could it be condensation from the humidity??