Best tip I ever heard was to involve all the senses. It's very easy to get stuck on discribing how a place looks, we live in a very visual world after all. But think about sounds and particularly smell, rarely do people discribe the smells of places but smell prompts memory more than any other sense.
Also try looking for some wordlists on google. Get a thesaurus and a dictionary.
And practice, practice, practice. Write out descriptions ahead of time, you don't need to use them in the game, but its nice to have some notes to look at. Read some published adventures, read novels, borrow or steal discriptions if you have to at first. Read a broadsheet every day or at least once a week.
The more you exercise your vocabulary the bigger it becomes and the easier time you will have describing interesting scenes at short notice.
The current issue of Dungeon (issue #114) has a two-page article addressing this very topic, with 6-8 descriptions of various environments such as forests, mountains, desert, etc.
Index cards and build your own. Just state the location then add single words to it, then look for alternative to each word in a thesaurus. Things to remember is describing something:
Lighting - first and foremost, it it well lit, gloomy, dark, shadowy, flickering with touch light. Distance & Size - one of the first things notices too, the size of a location sets the stage. Smells - Musky and damp, mold, dusty and dry, of a wet animal, of rot and death. Does the oder come overpowering, is a hint. Objects - What do you see, table chair? You can do this by walking into a room and making note of what comes you your eye first.
A picture is worth a thousand words, if you find a picture you like use it, this is great for breath taking wonders and will impress players to size and difficulty.
Edit: I prefer the great classics of literature. There are just so many of them I'm waiting to read. Personally, I read very few fantasy-genre novels. I have in the past, but have been disappointed often (although there are undoubtedly great pieces of literature in the fantasy genre).