1) I can hold these sounds for a while.
Both [z] (as in “zebra”) and [ʒ] “zh” (as in the middle of “measure”) are sounds you can hold for a while. Here, I’ll hold them for 4 seconds each. Try it with me.
• [z]…[z]…[z]…[z]
• [ʒ]…[ʒ]…[ʒ]…[ʒ]
2) They’re both voiced consonants.
Put your fingertips on the front of your neck, and make the [z] and [ʒ] sounds.