Here's  what  works for me:

I stand at one side of the room and throw  my keys on the floor, telling the CLASS to make me a sentence about what I just did and to BEGIN the sentence with my name.  I always get "Ms. VG threw her keys on the floor."  
   VG:  And the subject of the sentence was ??
   Class:  Ms. VG.
   VG: Right!  And the verb  
   Class:  Threw.
    VG: Right again.  Now--
I pick up my keys and do the same thing again, but this time I tell them they must begin the sentence with "The keys."   It  takes only a few minutes longer for them to get "The keys were thrown on the floor by Ms VG."   Then --
   VG: And the subject  is ?
   Class:  The keys.
    VG: Right!  And the verb?
    Class (this takes longer, several tries, but eventually        
            someone says it right):  Were thrown.
    VG : Right.  Now,  in the first sentence, was the subject
             DOING what the  verb  described?
  Class:  Yes.
  VG:  Was the subject ACTIVE, doing something?
   Class:  Yes.
   VG: Ok, how about the second sentence--did the            
           subject DO what the verb described?
   Class: (much more slowly!)  No-
   VG: Was the subject ACTIVE, doing something?
   Class:  No-
    VG:  Or was the subject  PASSIVE, just sitting there     
              letting something else DO SOMETHING TO IT?
    Class (very tentatively): Passive--
      VG:  Yeah--the subject didn't DO anything, but   
                somebody or something did something TO the 
                subject.  I don't know why we call the verb 
                "passive"; it's actually the subject that's sitting 
                there passively letting something happen to it--
                but that's the way it goes.  We say "was thrown" 
                 is a PASSIVE verb.

Another day,  I  use BODY-DIAGRAMING.  I  get 3 students up front and give them 3 slips on paper.  One slip says "The new outfielder"; another says "hit"; another says "the ball ."  Then I tell them to arrange themselves so that they make a sentence and that they must somehow interact with each other in so doing.  They do pretty obvious things, the subject usually hitting the verb with enough force to bump the verb into the direct object..  Then I get 3 more up--keeping the first 3 in place.  These three get "The ball"  and "was hit"  and "by the  new outfielder."   I give them the same instructions.  It takes them a few minutes, but they usually end up with a subject  and verb out front and the prepositional phrase a step or two behind them, with a hand holding onto the verb.

Then with both groups of 3 "acting," I ask the class to tell me the real difference in what's going on up there.  Someone will eventually get it--that the action goes to the right in one group and to the left in the other.

If I then ask them to look only at the verbs in the two sentences and find a difference, someone will eventually notice that the passive verb has two words.  And if that class has by then memorized all the "do," "be" and "have" verbs, I'll ask what family the helping verb belongs to and wait until someone recognizes the "be" family.  (And rarely, someone is far enough along to recognize the past participle--RARELY.)

If time allows, I get other sets of 3 up and ask them to make up their own short sentences --with active and passive verbs--and rearrange themselves as necessary.  We get lots of laughs, they find out how tricky it can be to shift from one voice to the other, and how indispensable the "be" verb and the past participle are.

ENOUGH!  Have fun...