A winning proposal must contain features, benefits and proof. It should have a win theme and be specific to the customer. It must appeal to all reader types and be presented in a clear and concise way. You can use this self-audit questionnaire to see if you are missing any vital ingredients in your proposals.
Preparing a perfect proposal is like baking a cake. If you include all the ingredients you are likely to be successful, but omitting one could be disastrous.