Possessing mastery of a language gives you instant credibility, and makes people take you more seriously. Of course, this does not only mean being able to speak well, but being able to write properly.
A place where people often get hung up in their writing is on abbreviations. There are quite a few kinds of abbreviations and all are acceptable in only certain settings and contexts.
The main type of abbreviation uses letters, such as initials. To abbreviate properly, you will usually leave out words such as "the", "of", "a", "an", "and" and "or". For example, the United States of America is not the USOA, but the USA. "People for the Ethical Treatment of Animals" is not PFTETOA, but rather PETA. Normally the "little words" will not be included as a part of an abbreviation like this unless they help spell an easy-to-remember word or name.
In many cases today a period between each letter is sometimes considered obsolete, so unless the name requires one or if you just decide to use them, your first period should come after the first letter in the abbreviation instead of before it. You may even see these abbreviations with only the first letter capitalized, particularly if the given brand has become a household name.
To pluralize an abbreviation, such as the SATs, using an apostrophe is actually a little-known error. The apostrophe rule actually also applies to decades put into Arabic numerals, as in the 1990s or the 50s.
And then there are numbers. A good rule of thumb is never to abbreviate the numbers one through ten when using Arabic numerals.
Units of measurement should not be ended in periods, because it is easy for some non-math-heads to confuse the periods with decimal points, especially when the handwriting isn't perfectly neat. You also don't need periods after periodic element abbreviations, in which the first letter is capitalized and the second, if any, is in lowercase. So whereas Ne stands for the element neon, NO3- is an ion of nitrate, with the two elements nitrogen (N) and oxygen (O).
Generally in business letters and English papers, you won't use don't...or any other contractions. Like the case with Arabic numerals, most teachers and employers want to see that you took the time, ink and paper to spell out words like "do not", "should not", and "can not". And avoiding contractions will also cut down on any confusion between the homophones "your" and "you're", as well as "they're", "their" and "their". Occasionally these may just sound better, so if you must contract to abbreviate, do so in moderation.
The most important rule for abbreviating properly could easily be considered saving the abbreviations OMG and LOL for your BFF Jill. This is basically because there is nothing proper about these abbreviations! Teachers and professors hate to see these and may dock you points for them, and employers will almost definitely hold these as unprofessional strikes against you that may cost you that great new job or promotion.