When you write, you are painting a picture with words. Writing is not a visual medium, yet your readers want to "see" what's going on, "hear" the voices of their favorite characters, "feel" what those characters feel, "taste" the delicious food you're referencing, and/or "smell" enticing or rancid scents that set the scene. How do you provide your audience with what they crave? Effective description.
Don't tell me, "she smelled something bad." Tell me, "the smell of rotting eggs permeated the room; bile lurched into her throat as she slammed her hand against her mouth."
Show me what you, as the writer, see in your mind's eye. What does that haunted house look like? How does that tempting candy taste? What does the vampire feel when his teeth penetrate his victims jugular? What does his victim feel? And not just "good" or "bad", which are subjective terms that I can attach anything to. Give me something far more descriptive and enthralling than mere "good" or "bad."
If it sounds difficult, don't get discouraged. Many writers have a difficult time with description, just as many have issues with creating effective dialogue. It doesn't make you a bad writer. You may have a great many strengths as a writer, but no one is perfect, and that's okay! Know where your strengths lie and take pride in those. Know where your weaknesses are and focus on improving these during revision.
When you're working on your rough draft, don't sweat it. The purpose of your rough draft is simply to get the idea down, roughly figure out where each part of your plot unfolds, and put those characters to paper. Don't worry, yet, about the quality of the writing at this stage.
Once you're done with the rough draft - that is the time to work through your manuscript and start revising your words until they paint exactly the picture you want them to. This would be the stage at which you need to start worrying about fleshing out those descriptions, making the dialogue seem natural but intriguing, and filling in the little things that bring your characters, settings, and story to life.
Is the street dark? How dark? Are there clouds covering the moon/sun? Are the streetlamps burnt out or non-existent? Description can be a fantastic way of conveying the way a character feels while sticking to the "show, don't tell" principle. Show me that the street is dark and foreboding, and I will naturally understand that the character is frightened. Describe the setting as bright and cheerful, and I won't be surprised to find that the character is quite pleased at the moment.
I found a wonderful article here (clicky) that gets into quite a lot of detail on how to write descriptions that are powerful, yet not over the top. While some writers have far too little description in their stories, others have a tendency to overwhelm their audience with a ton of over description, which can pull a reader write out of a story. I don't need three pages from you to get a clear picture in my head of how that rose looks; I only need a sentence or two.
One thing the article points out, and I will reiterate it here, is to avoid the use of too many modifiers. It's just messy when you say, "her long, beautiful, brown, curly hair was soft and silky." Pick a trait to focus on for a bit, "soft curls tumbled over her shoulders; she twirled one brunette strand around her finger." I'm not saying the second sentence is perfect, but it's a more effective use of description than the first, and it won't leave readers mentally exhausted from all the modifiers.
Do you find that effective description comes naturally to you, as a writer, or is it one of your weaknesses? How do you make use of effective descriptions in your own writing?