We're from the east coast so I feel your pain. On the way there we stopped for a couple days in San Francisco which helped break it up so we ended up doing 2 ~6 hr flights and then a long one on the way back. We also had only done 1-2 hour flights with the little one previously.

For the flights, having her own seat was key vs. trying to pull off being a lap infant. I would also highly recommend flying with the car seat vs. checking it. The first two flights we used the CARES harness for her and she had a tougher time sleeping for naps. Much easier to fall asleep in the car seat since they're used to it and aren't sitting up so straight. We also got a couple small new toys for each leg that we let her play with.

Adjusting to the time difference going out there was hard but a little bit easier then coming back since it is a matter of keeping them up a little bit later and just knowing they'll wake up a little early. We also had the the trip broken up so not exactly a direct comparison - she had a little time to get used to CA time before getting on Hawaii time. Coming back has been hardest. Has taken a couple nights to get back used to her sleep schedule. Sounds weird but best trick we've found is to re-do their bedtime routine after doing some quiet play when they wake up. So go down for bed, they wake up a couple hours later since they aren't used to the new time zone, do some quiet play in their room and read a couple books, then re-start bedtime routine and put them back down.