Demonstrate that you understand the buyer and the problem they're trying to solve, and that you have the right experience to help them solve it.
Use plain English, keep it succinct, and get someone else to proof-read it. You could get a designer to develop a document template for a more polished presentation, and include graphics and images to break up the text.
Your competitors may all have similar skills, tools and methods. Know what sets you apart, and clearly communicate it throughout your response.
The buyer wants proof that you know how to deliver services or supplies to meet their need.
Include examples of relevant experience where you have delivered something similar, and relate it directly to their requirements, in response to questions. Highlight staff expertise that's relevant to the RFx you are responding to.
Your relevant experience examples may also be your referees. Make sure you get their permission and warn them that they may be called.
Look at the weightings of the evaluation criteria and use this as a guide for what you should be focusing on.