A notoriously tough aspect for new cuers is assigning the correct handshape for /s/ and /z/. This decision occurs frequently in English because it affects plurals (e.g., cats, dogs, horses), posessives (e.g., Matt's, Pam's, and Rich's), as well as inflections for subject-verb agreement (hits, spills, washes). 

Rule

1) If the root word ends in a voiceless sound (e.g., rat), the -s ending is pronounced (cued) /t/. Exceptions to this will be noted below for voiceless sounds that need a vowel before adding the plural marker.

2) If the root word ends in a voiced sound (e.g., phone), the -s ending is pronounced (cued) /z/. This inclueds root words that end with vowels as all vowels are voiced. Exceptions to this will be noted below for voiced sounds that need a vowel before adding the plural marker.

3) Exceptions to the rule above are words that end in a type of sound called sibilants. While one does not necessarily need to know the terminology, examples of each follows: /ʃshshʃshshʃshsh/ (wish), /ʧchchʧchchʧchch/ (church), /sssssssss/ (bus), /zzzzzzzzz/ (buzz), /ʒzhzhʒzhzhʒzhzh/ (garage), /ʤjjʤjjʤjj/ (judge). These words take the vowel /ɪĭiɪĭiɪĭi/ or /əəəəəəəəə/ before the vowel, but do not take /ɛěehɛěehɛěeh/.