In working with students of various ages to improve writing skills, I have noticed three major things (small things, I might add), that make a HUGE different in writing: (1) creating a solid thesis statement, (2) utilizing various transitional phrases between thoughts and paragraphs, and (3) always bringing the reader/audience back to your main point, or thesis.
See the paragraph above? I used a solid thesis statement so my audience would easily be able to follow the organization of the paragraphs to follow. In order to create a strong thesis statement, one might ask, "What am I trying to accomplish in this writing? What is it that I want the reader to understand about the topic? What short phrase or thought can sum up the bulk of what I want to get across to the reader?" Answering these questions will lead you to the strong thesis you want to create, provide good structure and organization, and ultimately, improve your writing.
In addition to creating a strong thesis, it is important to have transitions between paragraphs. One paragraph should flow into another (see the first sentence of this paragraph), recapping what has been said and lead into whatever the second point in the thesis is. This helps the reader follow your thought process without getting lost.
Lastly, in order to improve your overall writing, always bring the reader back to your most important thought or point. Notice the first sentence in this paragraph did just that. Even though you know where you are going, your reader may get bogged down in all the information. Keep this in mind when writing and it will assist your audience in keeping focused.
In conclusion, creating an strong thesis, using transition phrases, and always bringing the reader back to your main point/thesis will all make you a stronger writer and keep a solid focus! Good luck in all your writing endeavors!