Save 'To Be' for Later
If you're teaching beginners - I'm talking about starting at square one - and you have some say in the curriculum, or at least the order of the topics you teach, then I've got a tip for you: Don't make am/is/are the first verb you introduce.
Sure, it makes sense for introductions. "I am Diego." "She is fourteen years old." "He is a mechanic." "We are from El Salvador." Plus, we use to be so often, and it's obviously really important in our language, why not use that as the first verb they learn?
The problem is that I've seen so many learners use to be when they're not supposed to; many will even include the verb in virtually every sentence, as if to be is a requirement for producing English. I hear sentences such as "I am go to school," or "I am like chocolate." I wonder if they think 'am' always has to follow 'I', or even if they realize they are two separate words. I can't blame them, since we drill in "I am" from the very beginning.
Mind you, this problem I see is not just with beginners, but even with many intermediate learners. It's so ingrained in there speech patterns that it's a difficult problem to break, even for the ones who can identify the mistake once you point it out to them.
So instead of trying to fix this mistake after it's taken hold in the learner's mind, why not try to prevent it in the first place?
After teaching various introductory expressions, teach some basic intransitive verbs. The benefit of intransitive verbs is that they don't need objects, so how can create simple sentences with just a subject and a verb. Some examples of intransitive verbs include:
Of course, the verbs you choose to teach should be ones you think you'll use a lot in your class. You can even apply these to jobs; say what they do instead of what there role is (for example, "I fix cars," instead of "I'm a mechanic.") This may be a good idea anyway for those learners whose job titles are difficult to pronounce or remember.
Anyway, by the time you teach them am/is/are just a few lessons later, they students are already familiar with sentences that don't have them. They'll understand that to be is another verb, albeit a special one (although they likely won't consciously think about it in those terms).