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 rules above are words that end in a type of sound called sibilants. It's isn't necessary to know the name of the type of sounds, but it is worht knowing which pphonemes are included: /ʃshshʃshshʃshsh/ (wish), /ʧchchʧchchʧchch/ (church), /sssssssss/ (bus), /zzzzzzzzz/ (buzz), /ʒzhzhʒzhzhʒzhzh/ (garage), /ʤjjʤjjʤjj/ (judge). These words take a vowe, /ɪĭiɪĭiɪĭi/ or /əəəəəəəəə/, before /zzzzzzzzz/**. Note that, inspite of spelling,  these final syllables do not take the vowel /ɛěehɛěehɛěeh/. 

** You do not need to commit to either /ɪĭiɪĭiɪĭi/ or /əəəəəəəəə/ for these plural endings. In reality, you are likely to alternate between the vowels depending on whether the word is cued alone or in a sentence and whether the word is stressed or not. For example, one may say the single word bushes as /{bʊˈʃɪz/, but they same cuer may pronounce the word with a schwa when it occurs in a sentence, The bushes need a good watering.