Correspondence execution style has significant differences, depending on whether it is of formal and bussiness or friendly kind.
Let's suppose, for example, that you need to write a letter to a certain institution or address with some application, declaration or request trough a letter.
The address should be executed in bussiness manner. It is disallowed to use diminutive-hypocoristic words, too long words or words carrying emotional connotation. A letter (if it contains request to certain person) can be started with the words: «Уважаемый(-ая)» /Dear ... /, and further you can uncover the matter of your request or informing. You can finish with the following expressions: «С уважением» /Yours respectfully/, «Заранее благодарен» /Thanking you in anticipation/ etc. Pay attention: these words cannot be cut – this would be a sign of bad manners!
The following examples are commonly used: «Прошу Вас» /You are kindly requested/, «Информируем, что» /Please be informed that/, «Довожу до Вашего сведения» /We are bringing to your notice/, «Предлагаю рассмотреть» /I make an offer to consider/ etc.
If document forms require presence of certain lexical units (professional words etc.), then, as a rule, they are already created in patterns. It is necessary only to write down declarant's personal information.
It is appropriate to use Russian colloquial language here. Letters can carry emotional connotation, word cuttings, increase of punctuation marks in case of need (several question marks or exclamation points, dots), presence of interjections are possible.
Russian people are quite emotional by their nature, and it becomes very illustrative in private correspondence. Depending on intimacy of connections with a person, the following words may be used: «Дорогая» /My dear/, «Милая» /Sweetheart/ etc. Letters are finished with the following expressions: «До скорой встречи» /See you soon/, «С наилучшими пожеланиями» /With best regards/, «Скучаю» /Miss you/, «Люблю» /Love you/, «Крепко обнимаю» /Hold you tight/ etc.
BUT! These recommendations don't refer to text messages or the Internet forums, the style of which should be discussed separately.