When writing up notes for each place in a game I always include the following:
Feel: The overall feeling of a place, festive, somber and so forth
Sight: Instant sights plus things that people may see after a couple minutes, or if they wait
Smell: Always include smells and many times connected to the feel and sight of a place
Texture: The stonework, the ground, the walls of a home, the overall feel of the place to touch, or walk on.
The cool thing is when one of those is DIFFERENT from the other. If you have a festive place with people dancing and talking and laughing, the sights match that as well, and the textures are of amazing festive things but the smell is dank, wet, wormy, earthy, and strangely metallic. Man you can get a group to really raise their heads and go, "Wait what?"
I find those things work for people to and again mixing it up can really make someone seem odd or memorable.
You can make any person, place, or thing super memorable by either explaining those things or explaining them and having 1-2 not match what would be expected.
Other things I do;
We have NPC interruptions but I also many times have 2 NPC's tell a story. Just like 2 friends telling me a shared experience I break it up so one NPC is talking about what he saw, and suddenly the other NPC says "And you should have heard that damn Troll roar when I shot him with the 45!"
I also have 1-2 questions during every NPC monologue. Questions that are leading and happen right in the middle of a description.
"I admit it has been years since I handled the shotgun but I felt like it was only yesterday. I don't know if you saw me but- actually a question sir. Have you ever tried shooting the Nexus 415 expulsion rifle while riding a horse?"
I also love natural occurrences. Heart attacks, coughing, sneezing, hiccups, earthquakes, kitchen fires, lighting cracking, forks falling because someone listening to the monologue was surprised, and so forth. I also break up monologues by having NPC's sometimes walking and talking at the same time, get bumped into, excuse a servent, sign an order form, but never ever standing.
Items:
I also love to throw in little bits around items. Items that look amazingly cool but really don't do as much damage as a person may expect or want. Many of my players will take their scary looking 22lr versus a 45 simply because it looks scary

When describing items I also add the texture, smell it gives off, feel of it in their hand, and the overall picture of it within the game world.
Enemies:
I make sure that all baddies in games have specific traits that are traceable despite them being nearby or not. Smells, tracks, particular carnage, forts they make. I basically try to make sure that only truly random things are random and that races and creatures all have their own ecosystems.