All Five Senses
People have five senses with which they perceive the world: sight, sound, taste, touch, and smell. The fun thing is though, if you tell the brain something in the right way it can provoke the sensory information even if the person isn't there. This is how people get pulled into a good book. Their brain disengages from the world and absorbs the information it's being given. You can use this in your game to make your settings and set pieces even more vivid.
Sight is the easy one and the one everyone uses because it is so common. We give visual clues all the time. Something is taller or smaller than something is often done by sight. Color is done with sight. Skin tone is done with sight. "you see 4 goblins up the road" is done with sight.
Sight is easy, but also easy to forget about when focusing on other senses. Use sight. It should be your most cited sense because it is the primary one we use so often. Just don't use it to the exclusion of something else.
Believe it or not we use smell almost as much as we use sight. The human nose isn't as sharp as a dog's nose, but it takes int he world around us. If you tell someone a place smells like a farm in summer, you'll notice anyone who knows what that smells like wrinkle their nose because it's not the most pleasant of aromas. Smell is something that is great to bring in and describe. What is the difference between a noble person and a peasant if you can't see them? Smell is a big clue. The noble person can afford to bathe and wear perfume. The peasant can't.
Some people have particular smells that tells more about them. A warrior may smell of steel and leather, while a wizened wizard may have a home that smells of old parchment and dry ink. A ranger likely smells of dirt and wood, while a bard on the road will have a faint whiff of stale beer and an enclosed environment from the taverns. Or you can be really specific and have a character have a scent unique to them. Samurai Champloo - an anime - has a main character traveling Japan looking for a Samurai that smells of Sun Flowers. That is the only distinguishing characteristic, but it is unique enough to work.
We often tell people they hear things, but that doesn't count as sensory description. "You hear him say
Touch & Taste
I combined these two because they're among the rarest to see, but they can be important. Touch can be all about texture, and you can give some touch with visual cues. "The wood looks smooth with no seams for the construction" gives a sense of how it feels. But you can also use temperature for touch. The heat of the sun. The cold of the wind. The way the ground feels against weary feet or a rock in a boot. Finally for touch, skin too has a texture that you can give because it can also often be seen even if it still describes feel. An old salt and wind burned sea captain, vs. the soft smooth skin of a noblewoman. The rugged, scarred flesh of the veteran soldier, vs. the heavy calloused hands of a farmer.
WHich leaves us with taste. Most smart PCs don't put random things in their mouth, but that isn't the only way to use taste. Smells can have a taste. The thick smell of dung also has a taste to it. Moisture can have a taste to it. The smell of a swamp has a taste to it. Then there is food. Food is so cultural and so location specific that if you can make a difference in describing food you can sell a place. Recently in my L5R game the players traveled through Crane, Scorpion, and Crab lands. The key difference? The food. Crane food was plain, but sweet with elegant dishes and tasty deserts. The scorpion food tasted like fire with spices and textures that set the unwary mouth ablaze. While the crab had plain fare with no real decoration, because they have no time for such things. It helped set all three clans apart, and fed into later RP about things that happened.
A Little Goes a Long Way
When using sensory description, it's important to remember that it is like seasoning or spices with cooking. You only use a little. A little goes a long way, but too much and you'll overpower the meal.
You don't want your players sitting there for 30-45 minutes while you go on and on about the details, running down a checklist of all 5 senses for every object. But you don't want them just hearing "you're in a cave, what do?" either. Go through the description, and use some sensory information lightly to spice things up.
"You taste moisture in the air, and smell a stagnant pool. The drip of water leads you deeper into the cold, dark cave" touches on all 5 senses and doesn't it quickly. It doesn't matter (in this instance) if the cave wall warbles or is smooth. The person gets a mental image from the sound, smell, and taste of water in a cave they can make little of because of the darkness and knows it is chilly.
Focus on key details, let their mind supplement the rest. You can bring up other details later if need be, but broken up around things the players are doing.