I try to boil everything down into the essentials of what the players need to know to start making decisions and then sprinkle in a few flavor elements if I can do it without taking too long.
For monsters, the essentials are:
1. General size, form and description. Man-sized humanoid, giant bear, hulking brute, dragon, floating cyclopean orb with eye stalks, etc. If it's armored I'll add that in so the PCs can guess high AC.
2. Current demeanor. Enraged, cautious, slinking about, resolute, gleeful, etc. This is an insight into the monster's personality, tactics and goals and should never be ignored in a description.
3. What it will use to kill the PCs. I usually end with the business end of the monster for the purpose of drama. Massive claws, slobbering maw, barbed spear, writhing tentacles, smoking nostrils, enormous bulk, etc. This is also where you emphasize that this thing is awful and terrifying and that, if the PCs kill it, they'll be awesome for doing so.
Other than that, choose specific and fun adjectives over generic ones.
If you're using a lot of the same type of monster in the same encounter (or series of encounters) give the important ones an identifying trait (big noise, one eye, scarred belly, rattling armor, unusually short, etc.). I'll make up a quick list of 10-20 such traits for use with orc bands and such that I can refer to as needed. I won't describe all 20 orcs or skeletons in an encounter, but calling out leaders and other notable members is helpful.
It's okay if you can't adequately describe something that's bizarre, such as extraplanar creatures that have no real world equivalent. "A horrid, pusalting thing festooned with seemingly random array of insectile appendages and eye clusters lurches out of the dark and, with a chittering laugh, attempts to grab you with its fishhook claws..." is fine. Players hear "ugly monster attacking us" and think "kill it with fire and steel".
Whatever the case, the thing will be dead in a few minutes anyway so its not really necessary to over-describe.