Dos and Dont's
Style guides and usage books don't agree where to put the apostrophe in dos and don'ts (do's and don't's? do's and don'ts?). Here's my recommendation.
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 we have 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 don'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. The Chicago Manual of Style and others recommend dos and don'ts. The Associated Press and others recommend do's and don'ts. Eats, Shoots & Leaves recommends do's and don't's
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.
Get more tips like this in Grammar Girl's 101 Troublesome Words You'll Master in No Time: