I know this doesn’t really apply to much of anything, but I’m going to post my opinion.
Carry a flashlight.
In your pocket, on your belt (my choice), in your purse, whatever.
In this day and age of unexpected emergencies, it can literally be lifesaving. Don’t be fumbling in the dark for your doorlock.
More commonly (especially as you age), you will find things you just can’t see. Maybe you can’t make out that instruction card with the small print. Maybe you can’t find the screw you just dropped.
Having that light close at hand can make the issue just "go away".
Many people will bring an umbrella with them if there is a 25% chance of rain. Most nights come with a 100% chance of dark. Where’s your flashlight? – Foursevens
There are many, many flashlights available. You can go to the local Walmart or whatever and get something that probably is decent. If you are really going to carry all the time, I would recommend something better, though.
I feel that many people don’t see the use, because "it’s always dead when I reach for it". I believe that’s mostly because you aren’t buying a decent light. Get a good, LED based light (practically never burn out, and dropping them won’t blow the bulb). Buy batteries for it and keep them changed.
There are lots of lights and sellers of "tactical" lights. If you are really interested, check out Candle Power Forums. There are more discussions and experts than you would believe.
For myself, I recommend the Sunwayman V11R very highly. Many of the tactical lights have multiple modes and features. That’s all fine, but I find the only thing I care about is light, and how much of it. The Sunwayman has an easy to adjust, twisting ring that just dims the light as required.
I’ll probably write more about flashlights and other EDC (Every Day Carry) gear I think everybody should have at hand, but for now… find a light and carry it.