To form the plural of a single letter, italicize the letter and add an apostrophe followed by the unitalicized letter s.
Recommended: We called tech support because the printer wasn't printing uppercase B's or lowercase p's.
Recommended: If you still use a pen and paper, dotting your i's is still necessary. Cross your t's, too.
When the capitalization is irrelevant, use uppercase.
Recommended: Mind your P's and Q's.