One of my biggest issues when I first started DMing was remembering NPCs. I'd have to make someone up on the fly, or forget a quirk for a guard the party had taken a liking to, and it always felt terrible to inadvertently give my group a peek behind the curtain. To compensate for my lackluster memory, I developed a template for NPC generation that's relatively short, sweet, and hits all the most important points for your standard non-player character. The template goes as follows:
This gives you everything you need to have a rough idea of how the character would act or react, plus you've got some juicy quest or gossip for any player that decides to invest in this little corner of your world. On top of that, it all fits comfortable on a 3x5 note card which makes them ridiculously easy to organize!
This method has been not only wildly helpful for me in terms of remembering details of my NPCs and keeping them organized in my head, it's also made it much easier for me to create more complex, fleshed out characters, which increases the chances that your players will become invested in the first place. It also serves as a straightforward template, so you can make up a character as you need them, write a name down on a note card, and just fill it out when you have 5 minutes to spare. It's really changed my prep game, and maybe it'll help you out too. Here are a few examples of my own: