Formal, solicited proposals are often created in response to a Request for Proposal (RFP). These requests usually come with clear instructions on what the client wants from a prospective vendor. This includes product and service requirements, qualifications, time frame, pricing, and examples of past work or success stories.
When you create a solicited proposal, the prospective client has already decided they need help for this project. They now must simply choose the best option available. This makes the proposal somewhat easier to write, since the client is already on board to agree.
Craft your solicited proposal as a more formal presentation, and include a slide deck with graphs, images, and case studies. A well-designed presentation will make your proposal stand out from the rest and help you present information in a clear, approachable manner.