Having flown dozens of long hauls (NZ is so far from everywhere, especially europe, that for me a 9hr flight is short!) here are my tips:

Get the best seat you can (making full use of status, early on-line check in etc). Ideally exit row, bulkhead or at very least aisle seat (even just stretching legs out into aisle makes a difference).

Try to get some sleep (depending on timing of flight). Some alcohol (not too much) and stay away from caffeine helps.

Noise cancelling headphones also helps me sleep by blocking out noise.

If you cant/wont sleep then good movie and magazine (one havent already started) helps fill in time.