Several items fit into this category:
Practice on a durable heli (Corona)
Use training gear to minimize the chance of tipping over and damaging the rotor blades and tailboom
Use a boom protector (dowels taped/tiewrapped to boom to reduce damage if main rotor blades hit the boom)
Practice on the simulator. The helicopters in the simulator do not require a trip to a hobby store or waiting for parts to arrive from an online store. They don't require finding a dealer with parts in stock, or waiting for parts to arrive, or installing the replacement parts, or tedious detective work to identify the cause of vibration problems.
Simulators and heading hold gyros are probably the two biggest advances in the last five years for reducing the learning curve of R/C helicopters.
Keep a supply of commonly damaged/lost crash parts. For the Corona, this would be: main rotor blades, tailbooms, vertical tail fins, and Z links.
For other helicopters, this would include: main rotor blades, flybars, flybar paddles, spindle shafts, main rotor shafts, tail booms, tail rotor blades, tail rotor shafts, vertical tail fins, and landing gears.
Know your limits for each day. Your limits for each day will change depending on various factors, including: how much sleep you've had, how tired or stressed you are, and other variables.
Therefore, you should start your practice each day by performing very easy exercises to warm up. This will allow you to gauge your limits for that day. If you are not flying well, then you should hone your existing skills instead of trying to learn new skills. This will reduce your probability of crashing.