Do not tell the reader that a room is filthy. Instead, describe the state of the room and let the reader draw his own conclusion.
This is not an effective description:
This small room is filthy, disgusting, and is home to a lot of bugs which fly around in a very annoying way.
Apply the 'show, don't tell' rule to this description:
This closet-sized room is littered with dirt and globs of decaying flesh. The walls are covered with grime, and a buzzing cloud of tiny winged insects fills the air.
It is a mistake to assume the reader has any knowledge of nearby room descriptions. This common problem occurs when writing several descs concurrently; it's easy to think of the descs like paragraphs in a continuing story rather than discrete chunks of descriptive text that must stand on their own.
This is a bad description:
The corridor continues to descend deeper into the earth. The air gets cooler and the once-faint background hum is now identifiable as the sound of rushing water.
Make the description stand on its own:
This long, straight earthen burrow gently descends to the southeast. The cool air is calm, but an occasional warm draft blows through the area from uphill to the northwest. Downhill, the rumble of rushing water can be heard in the distance.
It's tempting to dig deep into your vocabulary and dust off those big, fancy words that you learned in school. However, individual words shouldn't call attention to themselves. This is a desc that suffers from delusions of grandeur:
From the heavily-raked eaves of this abode dangle pendant spears of frozen moisture, dripping like the venomous fangs of a slathering beast.
Change it to this:
This house features a steeply-sloped roof that supports a row of dripping icicles.
Use the appropriate number of adjectives. It is always better to do away with unnecessary adjectives and overlong descriptions. Throw out your thesaurus.
The highly polished circlet of metal clasped tightly about her graceful neck blazed with a reflected fiery radiance that bespoke the violence of its birth in the forge-fires of a forgotten dwarven smithy as she turned her gaze to the rosy fingers of dawn that uncurled inexorably across the azure sky.
While there's nothing grammatically wrong with that sentence, I feel dirty just writing it. Here's a better one:
Her necklace flashed with reflected light as she turned to watch the sunrise.
Here's the same sentence with a few tasteful adjectives added:
Her silver necklace flashed with a warm blaze of reflected light as she turned to watch the autumn sunrise.
If at all possible, do away with the word 'you' in descriptions. Remember that someone viewing a room description might be peeking in from a different room, or scrying from afar.
In front of you stretches a long wooden counter. To your right is a window which looks out over a river.
This should be changed to:
A long wooden counter sits near the northern wall. In the east wall is a window which looks out over a river.