When Editing
Consider the age of the post.
Whenever a post is edited, it is bumped to the top of the queue on the front page. This crowds out other posts which are also deserving of attention. If a question is relatively old and already has several answers, it likely doesn't need the renewed attention—such a post should not be edited unless the edit provides a substantial improvement to the question (e.g. edits to correct a single spelling error should probably be avoided). On the other hand, smaller cosmetic edits to very new posts are more acceptable.
Make a noticeable improvement to the post.
After a question is closed for lack of quality, context, or clarity, it is automatically added to the Reopen Queue after it has been edited. If the edits don't improve the question so that it meets the site's quality standards, then the question is unlikely to be reopened through the Reopen Queue. As such, making small cosmetic changes to a closed question without fixing more substantial problems can unfairly punish the asker.
To avoid this, make sure that edits do as much as possible to clean up or correct a post. If you edit a post to add a tag or to fix the formatting in a title, also check to see if there are other issues which need correcting: correct spelling and grammar mistakes, improve the formatting of the mathematics, remove salutations, add necessary links, and so on. When an edit is completed, the post should be up to the quality standards of the site.
Adhere to the original post's style.
Edits which significantly alter the original style of a post are discouraged. For example, please do not replace
\lim_{x\to a}($\lim_{x\to a}$) with\displaystyle\lim_{x\to a}($\displaystyle\lim_{x\to a}$) or\lim\limits_{x\to a}($\lim_{x\to a}$) (or vice versa); don't change $\sin x$ to $\sin(x)$; don't Americanize or Britishify spellings (e.g. don't replace "color" with "colour" or vice versa); etc. Please also note that there is a general guideline on the site to avoid titles which stretch the vertical spacing. Thus the use of\displaystyle,\dfrac,\dbinom, etc is strongly discouraged.Write a clear edit summary.
This is particularly true for users who have not yet accrued enough reputation to make edits without approval, but is good practice for everyone. When an edit is made, there is a field for entering an edit summary, which should describe the edits being made. This helps reviewers to understand why you are editing a post, and can help draw attention to seemingly small changes. These edit summaries can also serve as guidance for the original poster, who might learn from the changes you make.
Limit the number of edits you make.
This is true both in a local sense, and in a global sense.
In a local sense, do not make many edits to a single post. As noted, every edit bumps the post to the top of the queue, and decreases the visibility of other posts. This isn't fair to the users who have posted these questions, so please try to avoid excessive editing. If a post is very complicated, you might consider using the Sandbox for drafts of long, complex posts on Meta. This can be a good place to get your thoughts clarified and the formatting worked out before making an edit (or posting a question or answer in the first place).
In a global sense, try not to edit too many posts at once. A good rule of thumb is that you should never bump more than one or two posts to the front page at the same time. After editing one post, give it some time to fall off of the front page before editing another. Or limit yourself to only four or five edits per day.
In both cases, the goal is to ensure that everyone has fair access to the front page.
When Suggesting an Edit
All of the above guidelines hold when suggesting an edit, only more so. While it might be acceptable for a grizzled veteran to make a small change to a very old post and leave a useless edit summary, new users who need to have their edits approved by the community should avoid this kind of editing. When you suggest an edit, you are asking other users to take some time out of their day in order to review your work. Please respect their time and suggest edits which make significant improvements, rather than minor cosmetic or quality-of-life alterations.