What's the Trouble? The spelling of do's and don'ts is inconsistent.
Generally, you don't use apostrophes to make words or abbreviations plural (e.g., CDs, 1970s, hats), but English has a few exceptions. For example, you can use apostrophes when they help eliminate confusion, which happens most often with single letters. Mind your p's and q's is the typical spelling, and we write that the word aardvark has 3 a's, not 3 as.
Dos and don'ts is an especially unusual exception. The apostrophe in the contraction doesn't seems to make people want to use an apostrophe to make do plural (do's and don'ts), but then to be consistent, you'd also have to use an apostrophe to make don't plural, which becomes downright ugly (do's and don't's).
Style guides and usage books don't agree.
What Should You Do? Unless your editor wishes otherwise, if you write books, spell it dos and don'ts; and if you write for newspapers, magazines, or the Web, spell it do's and don'ts. If you're writing for yourself, spell it any way you want. Just be consistent.
For more on using the apostrophe in plurals, see Erin Brenner's "Punctuation Point: The Apostrophe, Beyond the Basics."
Mignon Fogarty is better known as Grammar Girl. She is the founder of the Quick and Dirty Tips network, author of Grammar Girl's Quick and Dirty Tips for Better Writing, and the creator of the iOS game Grammar Pop. She is also the Donald W. Reynolds Chair of Media Entrepreneurship in the Reynolds School of Journalism and Advanced Media Studies at the University of Nevada, Reno.Click here to read other articles by Mignon Fogarty