When used in a sentence, a proper noun should be correctly capitalized and make sense within the context. A proper noun is a specific person, place or thing with a specific name. For example, a truck is a common noun, but a Ford F-150 is a particular type of truck and therefore a proper noun.
Proper nouns may not always follow normal spelling conventions, and it's important to maintain the name however it is spelled. The popular children's toy store Toys "R" Us, for example, requires both the substitution of the R for the word "are" and the double quote marks around the letter to be correctly spelled. Proper nouns may also have unusually capitalization, such as iPod, and are often trademarked. For instance, even when "iPod" is used at the beginning of a sentence, the "i" does not get capitalized.
Because they can be long and cumbersome to deal with, it is generally not a good idea to use proper nouns for every instance. Writers should vary their sentence structure and word usage by substituting common nouns and pronouns where applicable. For instance, with Toys "R" Us, writers may use "toy store" or "it" to vary word choice. It's important to always have the first mention of the noun as the proper noun form, however, to ensure there is no confusion over what any subsequent nouns and pronouns are referring to.