Gads but do I ever hate "rules"! But, you asked!
1. If the noun ends in an unvoiced consonant sound: /f/, /k/, /p/, /t/, /th/-(thin), pronounce "s" as /s/.
2. When it ends in a voiced consonant sound, /b/, /d/, /g/, /l/, /m/, /n/, /ng/, /r/ or with a vowel sound, /a/, /e/, /i/, /o/, /u/, pronounce "s" as /z/.
3. If it ends with /s/, /z/, /sh/, /ch/-chair, /zh/-the second "g" in garage, /dz/-(j), pronounce "s" or "-es" as /iz/.
In other words, if the noun ends with a sound other than the 5 unvoiced consonants, pronounce "s" with a /z/ (or with an /iz/ as the case may be).