Our post on ten phrases that are often misused because they were misheard has attracted lots of comments. Many of them related to errors where "of" gets used in place of "have", such as "should of" instead of "should have" or "should've". That is indeed annoying, but it also serves of a reminder of another useful rule: contractions are often best avoided in your writing.
I first got introduced to this principle by an editor many years ago, who argued that writing out contractions in full made for easier reading and comprehension, especially for people who had acquired English as a second language. It was a valid point: native speakers make enough of a mess of distinguishing between "its" and "it's" as it is, let alone when "it's" is used as an abbreviation for "it has". Spelling them out minimises the risk of confusion.
I have tried to keep that rule in mind ever since, even though sites such as Lifehacker often favour using contractions to avoid an overly formal tone. Naturally, that stricture doesn't apply if you are writing a novel, or in the kind of informal text, IM or Twitter chat where spelling and grammar often go by the wayside anyway. However, for work-related documents, it can be a useful approach.