1. Pack everything important in your carry-on bag.
My dad travelled frequently for business and my parents still travel frequently for pleasure. This is a lesson that is a habit for them and has rubbed off on me. For my race travels, that means my shoes, Garmin, and Garmin charger. Anything else I can buy. That doesn't mean I won't put my full race outfit and other supplies in my carry-on, but if space becomes an issue I could leave them out.
2. Download your Garmin data then delete your history.
I found out the hard way that your lap history can fill up in the middle of a race. Navigating seldom-used watch settings isn't something you want do be doing while you are running for a PR.
3. Don't pack gels in your carry-on.
Even though you might get through security, you could have to throw them out or worse miss your flight while they investigate the contents of your bag.
4. Document your running routine as a packing list.
Do you use glide or band-aids, eat homemade pre- or during-run snacks, listen to music? Write it all down before your last few long runs then make sure to pack everything you need. If it's too late for that, walk through your routine as if you are going for a run and put all the things you pick up in a grocery bag or something similar so you can make sure it all makes it to your luggage.
5. Lay out all your gear the night before.
If you are running close to home, his might not be such a big deal. However finding everything in a cluttered suitcase and unfamiliar setting if a hotel room can be tough especially before sunrise after a restless night. If you use pouches or bags, fill them up the night before. Put the gels in your pockets if that is where you keep them. If you are like me, you will still triple-check everything (and still forget something) but still better to stack the odds in your favor.
6. Don't forget your Garmin when you leave for the starting line.
I did that heading out to a half marathon. Luckily I had plenty of time to make the 20-minute round trip back to my hotel room, but that was stress I didn't need. Sure I could have just used the on-course clocks but doing math while running is harder than you would think.
7. Don't forget about the expo.
Most big races have them and they can be a great place to pick up gear and swag. More importantly, they can be a grocery store for gels and other last-minute needs (things I forgot to out on this list or forgot in spite of putting them in this list). So don't stress too much since you can find almost anything you might need.
8. Don't get new race gear at the expo.
Don't try compression sleeves for 26.2 miles if you've never used them. Don't try some new chewy calorie source when you trained with gels. And for God's sake don't get a new pair of shoes unless you lost or forgot yours and found a pair of shoes you've run in before.
9. Bring disposable clothes.
Most big races donate discarded clothes to charity so if you anticipate cold weather, bring an old sweatshirt, hat, gloves, whatever you think you might need. Keep in mind that your body heat will warm you up so you may want to discard these things at the start or right after. If you wear extra clothes, make sure your bib is on your bottom layer or on a removable number belt if you have experience with such things (but refer to #8 before you strap on something new).
10. Check in to your flight.
Typically you can check in 24 hours in advance. Get your electronic boarding pass or print it so you are ready to go.
So that is my personal encarnation of my running OCD. Hope it is helpful for you!