Use parentheses [ ( ) ] to include material that you want to de-emphasize or that wouldn't normally fit into the flow of your text but you want to include nonetheless. If the material within parentheses appears within a sentence, do not use a capital letter or period to punctuate that material, even if the material is itself a complete sentence. (A question mark or exclamation mark, however, might be appropriate and necessary.) If the material within your parentheses is written as a separate sentence (not included within another sentence), punctuate it as if it were a separate sentence.
If the material is important enough, use some other means of including it within your texteven if it means writing another sentence. Note that parentheses tend to de-emphasize text whereas dashes tend to make material seem even more important.