does it only tend to squeal when the brakes are cold? (common on hi performance pads). I notice my e90 does the same when brakes are cold on light applications. The only solution that may work is to drive the car harder.
Try to do a few harder than normal deceleration once the brakes have warmed up a bit. eg. 60km/h down to stop at a set of lights.. (don't do it when someone's behind you though!! watch for traffic behind).
Not sure what pad compound bmw uses but if it is a hi performance/ high operating temp pad then driving it in slow traffic and not using it hard enough over time will cause the pad and rotor to glaze over and start squeaking real bad. Sometimes if it's got to that stage the only option is to get new pads and ground/ machine the rotors then bed and use the new set "properly"