Spellcheck - A large amount of typos can be a real deal-breaker for new viewers, please spellcheck your edits preferably before submitting them, if this cannot be done or if you are unsure of the proper spelling, use an spell check website.
Fact > Opinion - Avoid making edits that use personal opinion that cannot be cited or corroborated from the manga itself(or other official works such as the Data book), this is important for the "trivia" and "relationships" sections of character pages especially.
Chronological Order - When editing a character's bio section, make sure that newer information is near the bottom and older material at the top of the article, flashbacks for example. Information added from the newest chapters obviously are at the last.
Redundant information - Avoid repeating statements or information that have already been added to an article, as it gets repetitive. This is okay if the repeated information is from different sections.
It's not personal. - If someone re-edited the information that you added to an article, don't get mad. Some admins/editors (myself for example) are a bit OCD about spelling, or grammar, or even how a sentence or paragraph reads, or "flows". Sometimes changes are required to make the content read easier. The next bullet on this list is related to this.
Summary - When editing, use the summary box to add notes to other editors of the changes you are making, this is more important if you are changing a lot of information, so that the other editors know why you did it, and it also lessens the chances of your changes being reverted/re-edited.
Proper Tense - For deceased characters be sure to use terms of "was" instead of "is" and etc.