I have a Focus 54reg 1.6 auto. It keeps blowing headlight bulbs - mostly the left one and usually on starting car or shortly after switching lights on. Have been using Bosch light recently - no difference. 4 bulbs in 2008!
When the bulb heats up , the filament is prone to breaking if there is excessive vibration. You should check that the headlamp unit is securely bolted and that the bulb sits tight in its seat and the spring retaining clip is actually pressing against the bulb base to keep it in place.