For numbers and noun coinages: simply add an s:

  • YMCAs, the 1920s, CPAs, lasers, PhDs

Single letters, add ’s: x’s and y’s, p’s and q’s, but all As

Italic plurals: put the final s (or ’s) in roman type:

  • I love the Rubaiyat’s lyrical poems.

Words as words: don't use an apostrophe:

  • His speech had too many ifs, ands, and buts.

See also: apostrophe