A successful writing proposal on Upwork should be clear, concise, and tailored to the specific job posting. The proposal should demonstrate an understanding of the project and the problem to be solved, and provide evidence of relevant experience and skills. To showcase relevant experience, it is recommended to include a relevant example of past work. It is also important to put oneself in the client's shoes and explain why one is a good fit for the job in two or three sentences. It is also recommended to check the client's reviews to get a sense of how formal or casual the proposal should be. Additionally, a well-written proposal should highlight what makes one unique and different from other candidates. Lastly, it is crucial to proofread and edit the proposal for grammar and spelling errors to ensure a professional appearance.