On site spares. Make sure you have (tested and shown to be working) spares for everything, and more than one for critical components and ones that are more likely to fail while you are restocking your spares. Engineers are easier to fly around than kit. Yes, it is expensive, and yes, it's a pain to have to store and keep track of the spares inventory (which always seems to 'go walkies'), but if you have a demanding SLA, it's the only way to go.