Use a ragged right margin unless you are required to justify the text. Remember that unjustified text does look less professional and more personal. Brief documents and those which your audience will read completely may be single spaced. This makes a document easier to scan. Also in longer documents, it is often a good idea to abide by the old saw of beginning paragraphs with topic sentences as this also facilitates scanning.
Listing can make information more accessible.
Use underlining, boldface, and italics for emphasis, but don't overuse. And remember to make headings parallel. Headings make it easier for your reader get oriented and to move around in the document. Information is more accessible because the readers know what section they are in, what to expect. Also the information may be more retrieveable because it's easier to return to certain sections.