Done at least a dozen TPAC (trans Pacific) flights just this year, lots previous. Biggest tips are:
-upgrade to Business/first if possible. Premium economy almost a minimum for the legroom...depends on airline. Economy doable of course, just check into what in flight entertainment options are and stickup on books/tablet/etc. depending.
-I wear a surgical type mask. The one I use is sold all over Japan, has a little slot it in for a moistened pad...really keeps nose and throat from drying out aside from the germ protection. Probably best thing I've figured out to do.
-slippers. Take off shoes, wear them instead while in cabin. Many Asian airlines provide free for various classes. ANA gives to premium economy, for example. Most airlines do for business and up.
- nice noise canceling headphones. Bose QC15, while pricey, help me relax a lot more without the background noise for dozen+ hours. Don't bother with cheap noise cx headphones, better served with plain earplugs than poorly functional ones. That said, some audio technics branded ones have worked adequately for me, but Bose ( a brand I do not care for otherwise) makes very comfortable, effective ones. Bring extra batteries.
-RE: neck pillow. I don't care for. I bring a small regular pillow which suits me. Do what works for you.
-cleaning wipes. Bit of a germaphobe, but I bring a couple single sized wipes to sanitize the TV controller, tray, armrest, etc. I know exactly how much (little) sanitation goes on with most airlines, unfortunately. 30 seconds of prevention, along with common sense things like hand washing, go a long way to keeping a stressed immune system from traveling from going over the edge.
-drink lots of water or juice. Limit alcohol to 1-2 drinks (I do wine or champagne with meal).
-comfortable clothing, but within reason. Sweat pants and a tank top, while comfortable, do not inspire respect from crew and fellow passengers in my humble opinion/experience. In extremis, the guy in the comfortablr slacks and polo/dress shirt might be afforded a bit more courtesy than someone dressed as a slob. I still consider flying a luxury or sorts, and dress at least decently. That said, dress how YOU feel comfortable...nothing to tight probably a good idea for long haul.
-bring yourself a small treat or two, or three!!.
Chocolate, cheese, whatever you like. Great to break out when delayed, tired, or just peckish between food service. Bring chewing gum, great way to "clean" mouth after sleeping or eating.
-fill out any requisite immigration paperwork early in flight before getting tired or during the rush while landing, taxiing, waiting in immigration, etc. FAs should be handing out.
-exofficio undergarments, or similar. Check amazon for them. Provide dryness and smell reduction for those 20 hour treks. Highly effective and comfortable.
Travel safely!