How to increase the lifespan of an Arduino.
You can't. All you can do is prevent damage from the elements or self induced damage from heat. Don't overdrive the regulator. Maybe add a fan if needed. Ensure water tight if you can for out door projects. 

The parts have a finite life based on their component quality. As you are likely buying cheap Chinese products, expect to get what you pay for. Any bargain bin capacitors are the most likely to go first. RTC crystal next, and probably the thermal printer after that. The SD Card based on how often you write to it.

Expect, in best conditions, five years.