There are many sources that claim to tell you how to more effectively write professional business letters, however there really isn't that much of a trick to it. Although you can pay for a course, download templates, or even buy software to download everything on computers to write professional business letters, really you don't need to do any of these things in my opinion. The internet has many databases of free writing samples, and you can download any sample of a professional business letters, see if you like the format, and then copy the style yourself, employing your own text.

Of course, writing professional business letters isn't just simply a matter of having a professional layout (although lacking a good layout will deffinitely sink your letter.) To write a professional business letter, or any professional document for that matter, it is crucial to learn how to be a good writer. Writing professional business letters is really no different than writing short stories, when looked at from a certain perspective. The key in both of them is understanding how to put words together to convey your message. 

If you want to write the best professional business letters, what you should do is read a lot. Not read samples of other professional business letters – I'm talking reading serious literature. I know it sounds crazy, and it certainly won't make you into a Fitzgerald over night, but the modern novel will develop your sense of language, which is your best asset in writing professional business letters. In the mean time, of course, you can practice writing professional business letters to friends and get feedback. If you can, get feedback from a friend who is a writer. Tell them that you want honest feedback – writers hate having to tell you that something is good when it isn't, so they will be more than willing to give the constructive criticism that you need to write better professional business letters. 

Of course, you can always take a course in writing. Sometimes, I've found, a teacher's skilled guidance can really make all the difference, whether you're learning to write professional business letters, or learning a new martial art. Most cities have many writing classes available, both at community colleges, and just independent of them, hosted by skilled professionals. There really is no quicker way to improve your writing than to study with an expert, and it can be a valuable way to make contacts as well.