Packing solo is almost the same as packing when you are going somewhere with a friend, lover or whoever. The difference is the fact that you have no one to blame if you forget to pack something, or if you don't have enough space in your luggage to take home that Moroccan rug you just had to buy. It's all on you.
The advantage is you won't have anyone that wants to invade your luggage with their excess oh-so-necessary clothes or Imelda Marcos shoe wardrobe when you are heading to the beach, or impulsive purchases en-route because it won't fit in their luggage. If you want to take 10 pairs of socks to the beach or that stunning night attire when on a backpacking jaunt, that is your choice and your problem.
Choose the luggage to suit your trip
Chances are you won't have a Sherpa to carry your bag from the train station to the hotel that may just be up the road. Think wheels on bags, a backpack, or whatever, but make sure that you can handle it alone, without the help of a minion running behind you with a bag on their head.
This will put you in control of your luggage and not the other way around, where you end up on a nightmare journey because you can't easily move with your luggage in tow. You are more mobile, you save money on excess baggage fees, and you can take an escalator, like normal people do, rather than getting into the elevator that is often used by those in dire need. Put yourself on a packing diet and only put in the things you really need.
Pack up your important travel documents
Not literally, but make sure you have copies of everything in a Dropbox, or email them to yourself. Paper copy two sets, one to have on you, and one to have in your luggage in case of an emergency, when there is no access to the Internet.
Take two credit cards from two different banks
There will be no one you can just turn to in case your card is lost or stolen, so take two cards from different banks. If one is compromised, you have another one on hand, without any drama. Be sensible and only ever carry one on your person when out and about.
Stash some emergency money in a totally separate place, as there is always a moneychanger somewhere.
Take two mobile phones
Your home mobile, plus buy a throwaway that you can put a local sim card into, as it's a bit of extra security.
Create your own medicine kit according to your needs
Such as an EpiPen for allergies. Definitely add in medication for diarrhoea, as it's hard to leave your room to go to a chemist if you need to be perched on the toilet all the time. Remember also to take medical evidence for any prescription drugs you need to carry as some may be restricted or even illegal in certain countries.
A global adapter plus a power bank
So it doesn't matter where you roam, you can always recharge your gadgets.
Odds and sods
Throw in a small flashlight (with batteries), a roll of duct tape for emergency repairs to shoes, bags or even clothes, plus a couple of plastic bags that always come in handy (think motion sickness in a bus).
A personal alarm
Even a whistle will do if it's loud enough—not just for the females, as guys do come unstuck at times in strange countries. It will attract more attention than yelling your head off if you feel threatened.
Not only can you read up on where you are, if you are not into a page-turning bit of reading trash, but it also gives you something to do other than staring at your food when you are sitting in a cafe or a restaurant alone. Actually it can just be a prop if you like, whilst you surreptitiously gawk at everyone around you.
The usual guff
Think undies or whatever you would normally take with a mantra of "be practical and take basics". Put it out on the bed and then take half of it away, but leave a sarong, as it can be turned into almost anything, from a bag to a dress and a towel to a picnic rug.
Pack an inquisitive mind, take your impulsive attitude and always have a Plan B.
Get out of your boring daily at-home routine: eat and sleep when you want to, talk to the locals (you can always walk away if it goes sideways), stay a long time on that mountain top or in that temple in quiet reflective silence and do things that make your heart go pitter-pat.