Not only does this mean that you should focus on neutrality in your letter’s appearance (avoiding the use of colorful paper, brash logo designs, and artistic fonts), but it also means that you should employ a very low_key, unnuanced, and professional closing phrase. The best_case scenario is that the hiring manager, colleague, or connection won't even notice the closing.
Then you arouse their desire by explaining how you can do this — for instance, by showing how successful you’ve been in other positions, or by explaining the specialized training you have relevant to the job. Avoid jargon here — explain clearly and concisely the benefits of the solution you offer. When I worked in marketing, I learned an important lesson: nobody buys anything based on features (and “buy” is used in its loosest possible sense, like you “buy” an idea). You have to explain exactly how those features will benefit the potential buyer.