Motivation is a tough issue.
I think if what you're giving them is so abstract that they can't see the connection between learning it and how they're going to apply it. Not only will it not be retained by that student, their eyes will roll back and they're gone before you start. No matter how difficult that concept is, if it's in math, science, or whatever, if the teacher can make a clear connection to how this relates to real life and take some of that abstractness away from it. Oh, without a doubt. If you can't, then you shouldn't be teaching it.