When writing personal correspondence is in the form of a letter or email, you know that you don't have to worry quite as much as you say things. You will always have good spelling and grammar decent, of course, but your personal friends and family are not going to nitpick and disown you because of some errors. However, when it comes to write business letters, you want to make sure they are perfectly formulated and totally without error. On top of that, it must be clear, to the point and as short as possible.
Business letters are all business. You do not want to send anything that sounds too personal, but I don't want to seem too stiff with someone with whom they did business with for a long time. Always begin and end with the correct terms such as "Dear" and "sincerity", though there are other terms that are acceptable. Look for the correct formatting for business letters, and never stray from that formula for the best result for each item. There are a few different formats that work, but more formal works best, even when you feel that the business is more casual side.
When it comes to the content of business letters, you want to make sure that your point is clear and that she said what needs to be said in words as few as possible. Skip the flowery descriptions, they may alienate some people. When you write a letter to the business on a particularly touchy topic, you want to write some drafts and then put it away to watch over again in a few hours or even the next day if possible. You will always have a fresh perspective so you are sure that you said what you have to say in the best possible way.
Finally, check, check and triple check spelling and grammar. If you are unsure, look up. A few moments that it takes to make that can mean a huge difference in what happens after the business letters arrive at their destination. Spell check on your computer is great, but will not catch everything. If you need to read the letter back, sentence by sentence, so that you can take any problem. Better yet, have someone read it otherwise. They have fresh eyes and catch problems that you did not.