What are the key elements that make a successful project proposal?

Two things to be consider :

-The well design of the project and the document ; it is important  before writing the document to include different stakeholders when outlining  or designing the project. Writing the documents either by following a format or develop  structure that contains clear justification and uniqueness of the project and the work to be done.In addition ,to show how you are going to do it and  with how much .  

The other thing to  achieve success after having a concrete document  is marketing. Few proposal passed . So carefully study the  market while designing your project.