My quick and dirty rules on in-game description.
Use all five senses
Don't just "describe a picture." Not every time - but remembering to toss in sound, a smell, a taste, a feeling, adds a lot more. You want the players to feel like they are there, and "the sun is bright in the sky above a sandy beach" is less immersive than "the sun beats down on you as you slog through the sand of the endless beach."
Also, remember to include motion. Movement of waves, grass, people. etc. reminds the players at an instinctive level that the world is living and breathing and going on even without them interacting with it.
Don't just tell them "it's raining" or "it's hot." The rain stings your skin, or there's a pleasant warmth making the smell of the foliage rise up around you.
Keep some brainstormed adjectives around and nice turns of phrase you want to use sometime. I frequently will see some weather or landscape or similar and write down a 2-sentence description to use at some later time.
For clarity, though, don't go crazy with the adjectives and adverbs. Concentrate on the verbs and put important things either at the end or the beginning, don't bury them in the middle (of a sentence, or worse a set of sentences). Remember that actually conveying information is the goal here, and PCs can be cross about having to ask "Wait, what do you mean it's on fire?" even though "you told them..." in the middle of some big ol' infodump. The classic mistake of reading a paragraph of boxed text about the inlay on the fireplace before you find out there's a crazed owlbear in the bedroom is unfortunately common. Lead in with the most important and notable aspects, and bring the rest out over time. A PC looking for situational advantage can always ask for more room description past "owlbear!" as needed.
This is a game - so there are some things that if not mentioned, aren't just lost color but will P.O. the PCs. So many folks don't bother reviewing stat blocks (and cast buffs and whatnot, esp, in prewritten adventures) and note that this ogre is wearing platemail and has an ioun stone whirling about his head. That's "you should have told us that 101."
It can be better to choose the one right word rather than layering on other words. Adding more adjectives to "I swing my sword" is worse than finding a more specific, vivid single word to replace "swing."
Be vivid but not lurid
It's easy to go overboard. We are playing through the Carrion Crown adventure path right now, and in an attempt to make it all "Gothicy" they go way over into purple prose on the room descriptions. "[5 sentences of description...] Whispers skitter like bats' wings through the darkness..." Those who didn't catch all the boxed text are confused about "there's not bats?" for five minutes while we and the GM roll our eyes. So describe, but get to the point quickly. You don't describe in an RPG the same way you'd describe in a novel. Readers have no choice but to wait till James Michener's happy ass gets done describing the turtle crossing the road, but your players generally would like to do something themselves.
Also, try not to info-dump. Rather than five sentences of "boxed text," give them one and then intersperse the other four into resultant descriptions of the PCs interacting with the world; this makes the additional description seem like a reward, not a startup cost.
OK, with the previous point in mind, it is good to use figurative language and metaphor, it's just so easy to go overboard or to trot out all the usual cliche ones. But this is how you also use the description to set a mood, from jovial to horrific. "The darkness watches you from three identical doorways" indicates to PCs to increase their pucker factor above "There are three identical doorways; you don't see light coming from any of them."