Seriously, get some great comfortable shoes for standing/walking. If you haven't in a spell, treat yourself to a nice shoe store, get your current sizing, and get some great comfy cushioned AF shoes and break them in before you fly out. You're gonna be on your feet for 20+ hours.
Other tips I generally give people visiting vs "working" E3:
-Bring your own portable hand sanitizer. Use that shit after EVERY demo, meal and general sense of desire to feel less grody.
-Don't eat at the convention floor, regardless of the convenience or how hungry you are. There a ton of decent restaurants/grab a bites in ten minutes walking distance. You will need to wait an hourish for a table at anywhere popular during the lunch rush, but plenty of great fast cheap options in the area as well.
-If you're going with a group of people, shot call your meet up place at the west entrance lobby and not a booth/area for any of the games in either of the halls. Shit gets crowded fast & the lobby of the west entrance is like custom built to find people fast. You will get split up and lost & it's part of the fun.
-For the past two years, both Bethesda and Microsoft have had reservation-based lines for some of their stuff for the public, and Sony did that for VR last year as well. If you are going to beeline for one specific thing on the first day, IMO i'd recommend making those appointments ASAP, if any of their stuff seems interesting.
-Most of the best stuff playable on the floor usually means a 40 minute to a 2+ hour wait in line, no exaggeration. The BotW line was 4 hours iirc. If you're going in a group, phone games that also have local network play are great to kill time with AND socialize with people around you! If you're going in a duo or solo, do what you think is gonna hold you down best.
-Don't be one of those assholes on the floor talking shit about demos, shows, sparse/vacant booths, or general console war dipshit etcetera. You're gonna be in ear shot of a lot of people who worked their fucking asses off just getting to the show, with many of them doing 16+ hour a day interviews, people handling and general showing off something they put a lot of labor into. Constructive criticism is fine; you're not required nor likely to like or love everything you see. But being a snarky butthole in line about some game or trailer or whatever is usually just draining for everyone that surrounds you, and there's a high probability chance someone who worked on said thing is in close proximity. Gaming events, especially E3, are more enjoyable when people are focused on sharing the things they love or are surprised by than punditry or anger.
Finally, not sure if they are doing it again this year, but last year the YouTube Gaming group had this fantastic area just a five minutes walk away from the convention doing hourly panels and Q&A deep dives on new/existing titles from actual developers open to all E3 badge holders. It was super duper comfy, really chill, and a great place to take a break and decompress from how hectic and crowded the show floor was. If you're kind of introverted like me and like having stuff you can just sit and relax to, that YouTube video panel thing was quite excellent and I highly recommend it.