1) Infantry clearing the area; this can be via "recon by fire" a technique where you fire at likely hiding places. This would be prior to movement of the main body into an area, and prior to fire of the AT missle.
2) Speed and shock of advance of the armoured force, which induces fear in the Milan Operator making him lose his aim or making him want to escape.
3) The Milan is a guided weapon where the operator still guides the missle to the tank, fire at the operator once you see the launch signature will make him move or duck and lose his aim
4) Finally not sure about the latest versions of the Milan but the frontal armor of the latest tanks is proof against all but the very latest ( and themselves quite expensive) AT weapons)
