For the big trips, I get very methodical. I have master lists of all gear, then check off the ones for the specific trip, thereby creating an individual trip sub-list. I test the gear and set it aside in a specific room. That's two sets of gear with thermal protection. I set aside all clothes and toiletries for the trip. This means I have duplicate items for the trip that I do not use on a daily basis, e.g. razors, deodorant. All that is left is the complex list of a camera and video rig with all accessories, computers, portable hard drives, earphones, Ipads and pods. This doesn't get packed until two days before and sometimes the night before. If all the camera bits comprise a list of 72 items, then I divide a table into 72 sections and lay it all out. Then it all gets packed at once. The only way I forgot anything is to look at the table and see it still there. I'll even have the list marked so I no where I packed it. I'm not that detailed on the way back.
Crazy? Anal? OCD? Paranoid? Maybe, but we, collectively, have left behind an incredible number of things over the years. Some minor, some not so minor. I dread forgetting things. Packing early helps lower my travel stress level.
I start on photo gear about a week before to make sure batteries are all charged and everything is in working order. Dive gear starts getting laid out a couple of days in advance. Clothing and other stuff goes in the bag the night before. Check lists are getting more and more essential as I get older!
We found that the check list is essential for the return. We left our wet suits in Guanaja! Thanksfully the guy running the resort was a Texan! 20 days and counting.
I actually have a check list for the grandparents who stay behind as well!