Fit key-operated window locks to all downstairs windows, those which can not be seen from the street and easily accessible upstairs windows. example: those above a flat roof or a drain pipe.

Even small windows such as skylights or bathroom fanlights need locks. A thief can get through any gap larger than a human head. Remember to move keys from locked windows and to keep the out of sight in a safe place.

Louvre windows are especially vulnerable because the slats can be removed easily from the frame. Glue the slats into place with an epoxy resin, and fit a special louvre lock. Better still replace them with fixed glass.If you are replacing windows consider laminated glass

As a last resort, consider fitting security grills to vulnerable windows.