Most of you have probably noticed that undercuts have been super popular the last few years. The first trend was the side shave, and then it was the nape aka the spot between your neck and your ears. 
At this point though, the undercut/side shave may have run its course and you are ready to grow it out. There is one simple "rule" to growing out an undercut if you ask me, and that is to shrink it gradually. I myself grew out my mohawk side shaves this way, and I have also done it for clients. 

As demonstrated in my poorly done drawings, you will not grow out all of your undercut at once; rather grow out small slices or sections of it. The effect is a much more subtle grow out. It minimizes the awkward phase, but does not eliminate it, for those of you who were hoping for fairy-tale magic. You WILL experience awkwardness and frustration growing out an undercut. 
That being understood, you can further minimize this awkwardness by using gel or pomade to smooth out the sections you are growing out, when they are about 2 inches long and therefore look fuzzy. Don't like that idea? Use bobby-pins.

Another option is adding tape-in extensions, like I did, if you are ready for the maintenance as well as the bill. You can put extensions in as soon as your hair is 2 or 3 inches long. They are the closest thing you will get to fairyland magic, as they do a beautiful job in making you forget how ugly your hair is. I faked having sides for at least 8 months!
Eventually your undercut will grow out to look a lot like layers, and even though that hair may not be as long "it should be", when you get to that point it's a cakewalk. Take it from somebody who knows.