I LOVE to travel but I hate to (un)pack so this is one of my tricks for avoiding forgetting something important and to cut down on packing time. I keep certain items always stay packed and ready.
In Flight Bag - In this I keep my earbuds, phone, passport, some cash, hand cream, a few snacks, in-flight beauty like lip balm, eye drops, touch-up powder, mints, lip gloss.

Earbuds - I keep mine in this little pouch charged and ready.
Chargers/Backup Batteries - I'm sure you have spares like me so it's good to keep one in your luggage. I keep two, one small one that I got at J.Crew for actually talking on the phone because it's cute, discreet and cordless but the Gold Cylinder one for the flight which holds three charges (mine is from Nordstrom but it doesn't seem to be available anymore so the link is for one that looks similar but it's from walmart.com. If you are an iPhone user like me, you can appreciate the short life of our batteries and when you are traveling you don't want to be that girl hovering the only outlet at the airport so that you can sit in an uncomfortable position on the dirty floor to watch the latest Snapchat.
Travel Toiletries Bag- It's important to always refill your travel sizes before you put your luggage away.
Travel Socks/Slippers- This could be 'me' specific but my tooties are always cold so I like to keep a pair of these Fit Kicks ready so I can stay warm while I'm on the road. At home I wear big heavy Uggs but when weight is an issue, I also keep these packed because I wouldn't be caught dead walking on the hotel room carpet.
Necessities Bag - For me this is a small ziplock bag chalk full of wipes! Shout wipes, nail polish remover wipes, deodorant, feminine wipes and a first aid kit.
Umbrella - One of the best purchases I've ever made was this umbrella, two weeks before I left for Europe (it rained 9 out of the 10 days I was there to be exact). It's so tiny yet so sturdy and I love the color!
Flat tote - Keeping something light and compact for when you do a teensy bit of unexpected shopping is so useful when you don't have to pay that extra weight fee when you might have gone over a pound or ten.
I've been doing this for years and since packing perfectly for a trip can be daunting, Believe me when I tell you it makes the task a lot more bearable, not to mention you never get to the other side unprepared.
Bon Voyage - Kara