You need to clean out the felt strip that contacts the window. When dust and dirt build up, they leave deposits on the window causing sqeaky squeaky.

 

mine are already squeaking again. i even removed the door panel and cleaned out as much of the felt track as i could get to however part of it is blocked by the structure of the door bracing and the fact the to window up or down is always in a certain portion of the track. the only way to really clean that part that i could think of would be to remove the window completely and use a "brush on a stick" approach.
it also doesn't help that my work parking lot is next to a place where dump trucks drive by leaving my car covered in a light coating of dust at the end of each day.