A trick about writing copy with good density is keeping the basic rules of how to position your keywords. Always (when and if possible) have your main keyword once in the title, once in the first paragraph, once in the body and once in the last paragraph. After that, you can just let other instances of the primary keyword, as well as secondary and tertiary, just fall where they may. Once you can make it flow naturally, you have something people looking for that keyword will get hooked on and read further!