One of the most common mistakes in terms of pronunciation for Mexican students is the difference between B and V. These sounds are not distinct in Latin American Spanish, and so the hardest task is distinguishing the two sounds to begin with. There are two things to teach here: position and voicing.
Basically, B is in the same position as P and V is in the same position as F. So, we really have to teach the difference between B/P, and F/V. This is where voicing is important. B and V are voiced sounds, and P and F are unvoiced. I think the trick is to build up to this with minimal pairs. Try 'ban' and 'pan' to get a feel for the difference between the voiced B and unvoiced P. Next, try 'van' and 'fan' for V and F.
This can take a while, and it helps a lot if you've already introduced the idea of voiced and unvoiced sounds. However, it is possible to teach the difference, and I believe that breaking down the problem in this way makes it easier to conceptualise. Remember, our first goal is to make sure our students can distinguish different sounds before they are able to produce them. If they can't hear the difference, they won't be able to pronounce it.