Creative concept
Developing your concept from research and thinking about initial ideas, to a well conceived written proposal takes time and effort.
A helpful tip might be to draw the concept - as a spidergram/mind map or record your thoughts on a dictaphone. This will help to flesh-out ideas, and articulate them in a way that is external to you. Pushing your ideas through such processes helps to clarify them and hone them into a form which may be easier to communicate.
Do not be afraid to use simple language. Language should be natural to you and the concept you are communicating. You should therefore write in a way that expresses your ideas. You may find you write best when your personality can come through.
Always remember that not all the people reading your proposal will have an arts background so make sure it is comprehensible for everyone.
Keeping references, footnotes, and appendices for more detailed, complex, and theoretical information.