Multiple barriers/fences/sensor fields around the runways and a great enough distance between the barriers and the apron to make it virtually impossible to defeat them all and sprint the distance to the holding jet in under 1 minute eg. about 1/3 mile secure perimeter with no cover.
Also, strategic deployment of some of those non-lethal technologies we keep hearing about, such as emission of low-frequency sounds that disorient and cause nausea...a few speakers around the end of the runways, pointed away from the aircraft, should make it very uncomfortable or impossible for anyone to lurk in the bushes.
Might even get rid of dangerous bird colonies and help avert birdstrikes.