Although keyword density is important, avoid keyword stuffing your articles particularly with too many occurrences of the same keyword. It seems that the single most common reason article directories reject articles is keyword abuse. Your articles may also be rejected if your resource box does not conform to the rules set by that particular directory. Get to know the terms and conditions of the article directories you use and stick to them.
Some directories will accept articles with as little as 250 words whereas others want 700 words or even more. Rather than writing specifically for one directory, you can write several short articles or around 250-300 words and then join two together to make a longer piece if necessary.
Once you get into the habit of writing in a style and format that is acceptable to the top ranking directories you will find it very easy to write numerous articles that will get accepted right away.