Recognizing the difference between active and passive voice doesn't come easy for some of my students, so I broke it down in a simple, visual way that has helped tremendously. The terminology of parts of speech can throw them off, too, so I use the words that make sense rather than the technical terms (although I teach those, too, for accuracy's sake.) To illustrate active voice, I draw this guy on the board:
Andrew is eating the burrito.
I explain that we have the agent (or do-er) of the action first (Andrew), and the thing that is being acted upon second (the burrito).
For one passive voice construction, I draw the same image, only flipped, on the board:
The burrito is being eaten by Andrew.
Now, the thing that is being acted upon (the burrito) comes first and the do-er is second. For the second passive construction, I just draw the burrito:
The burrito was eaten.
Now, there is no do-er of the action. So for any simple sentence, I ask them to visual a picture according to the order laid out in the sentence, and unless the do-er of the action is first, it is a passive construction. By getting this simple foundation, the students are ready to move on to more complex construction and higher concepts.
Here's the lesson plan I put together on this, which includes the above, plus some worksheets and a quiz. Now, I'm going to eat a burrito!