r's but Rs - handy on plurals of abbreviations.

Letters, abbreviations, and numerals. Capital letters used as words, abbreviations that contain no interior periods, and numerals used as nouns form the plural by adding s.

    * the three Rs
    * the 1990s
    * IRAs for retirement
    * vol., vols.
    * URLs
    * ed., eds.
      BUT:
    * p. (page), pp. (pages)
    * n. (note), nn. (notes)
    * MS (manuscript), MSS (manuscripts)

Use of apostrophe. To avoid confusion, lowercase letters and abbreviations with two or more interior periods or with both capital and lowercase letters form the plural with an apostrophe and an s.

    * x's and y's
    * M.A.'s and Ph.D.'s (or MAs and PhD's)

Words in quotation marks. The plural of a word or phrase in quotation marks may be formed by the addition of an apostrophe before the s, with the closing quotation marks following the s (though rewording is usually a better option). A plural ending should never follow closing quotation marks.

    * How many more "To be continued's" can we expect? (not "To be continued"s)


Noun coinages. Words and hyphenated phrases that are not nouns but are used as nouns form the plural by adding s or es. To avoid an awkward appearance, an adjustment in spelling (or sometimes an apostrophe) may be needed.

    * ifs, ands, or buts
    * dos and don'ts
    * threes and fours
    * thank-yous
      BUT
    * maybe's
    * yesses and noes (or yes's and no's, especially if maybe's is also used)


Source: Chicago Manual of Style, 15th edition and ProofreadNOW.com