It's best to avoid using and/or, but what should you do if you have to use it?
Kelly asked whether and/or in a subject makes the verb singular or plural. For example, what verb should she use in a sentence like this:
This message and/or attachments [is? are?] confidential.
You'd be hard pressed to find a style guide that doesn't admonish you to drop and/or and rewrite the sentence with just and or just or.
If you feel you must use and/or, my nonscientific survey of professional writing shows that you probably want to treat and/or as though it makes the subject plural. For example, Kelly's sentence would read
This message and/or attachments are confidential.
Usually, rewriting the sentence with or better reflects the meaning you're trying to accomplish with and/or, but sometimes people try to add clarity by adding or both to the end of the sentence:
This message, or attachments, or both are confidential.
In Kelly's case, a slight rewrite with and is probably the best choice:
This message and any attached files are confidential.
Get more tips like this in The Grammar Devotional: