Although it's more difficult to do so, BMW windshield wiper refills should be inserted from the bottom of the assembly, or going away from the locating point of the arm. Inserting them from the other direction will encourage them to work loose and come out.
When parking the car, always shut the wipers off before shutting off the car. This will ensure that the assembly is not subject to unnecessary strain when restarting. The system is made to withstand the starting surge from a certain position, namely the parked position. Moreover, if the blades are left in other than the parked position, the wipers will start to operate the moment the ignition comes on, a battery drain you don't need when trying to start the car. The consequences if the wipers have been frozen to the windshield boggle the mind -- they range from destroyed wiper refills to burned out wiper motors.
Author: Michel Potheau