Be aware of who your "target audience/reader" will be.  Make your writing sound conversational. 

Take care to avoid sounding stuffy, stiff, or uncomfortable. This doesn't mean you should use slang or inappropriate language; it means you should try to connect with the reader in the same conversational style that you would if you were in the same room with him/her having a conversation. 

Be careful not to use technical terms or jargon that will confuse the reader.  If there is any doubt as to whether or not your reader will understand a term, define it.  If there are numerous terms, use a glossary at the end of the document.

Visualize your reader as you write.  This will help keep your writing conversational.