Hey, Tatsuo,
I feel you on this one, since description is something that I never feel I get right. With that in mind, take what I'm about to say with a grain of salt - I'm not under any delusion that what works for me will necessarily work for someone else; but, hopefully, working through the exercise will be informative.
Why are you writing a description? Is there, in fact, a good reason to describe what you're trying to describe? Sometimes, the answer is obvious. If an object is significant somehow, it must be described; and a description about a person can say a huge amount about them, just as we try to figure out people by their body language in real life. Descriptions of places are important to creating the right mood. However, the temptation to describe everything, relevant or not, is real. I think this has gotten worse in recent times, when movies and TV are facts of daily life; it's tempting to want your reader to visualize the scene exactly as you do, but that's impractical at best. Sometimes, less is more. So, my first piece of advice is to decide what you want to describe, before you decide how you want to describe it.
Once you know what you're going to describe, you'll want to decide what the description is supposed to accomplish. Are you trying to create a mood? Do you want to imply something about a person through their appearance, or about an object's owner? Are you trying to reflect the viewpoint character's mindset? Once you know that, you can figure out what the object actually looks like. This process may take several iterations. Suppose you decide you want to create a mood, but the descriptions you come up with aren't quite working. Maybe you need to focus on different objects, or perhaps you may even need to add or remove things from the scene to give yourself the right tools to work with.
I'm sorry, let me amend that. What the object looks like is part of it, but it's always worth considering other senses as well.
On another note, there are some descriptions that are best on their own, while others can more easily be worked into other prose. Mentioning that Joe sits at a rugged wooden table is sufficient description of the table, even though it's not really about it. Likewise, a character gagging at the repungent odor of death tells us something about both the scene and the character. Dipping into "descriptive mode" briefly is a great way to give the reader an idea of the scene without having to write full-blown descriptions. Conversely, if the description really warrants its own passage, commit to it. If you find yourself running out of things to describe, you may need to revisit the decisions above. In other words, if you realize you don't know enough about what you're describing, put the pen (or keyboard) down, lean back in your chair if that's you, and try to get a better handle on it. Bashing your head against it is a recipe for frustration, but taking a step back to regroup works wonders. You might even put a note to yourself to come back to the description later and skip it.
Lastly, a description tells us something about the describer. This works better in first-person or third-person limited narration; fortunately, roleplays tend towards the latter. Thinking about how your character views the thing being described may give you another angle to attack the description from, especially if it's an unfamiliar or fantastic thing. I've personally gotten a lot of mileage out of clumsy circumlocutions when a fish-out-of-water character sees new things.
I hope some of this helps.
