Start enough in advance, and then don't rush it. That's the key to teaching bar mitzvah boys successfully.

They will be nervous enough about "performing" in public. There is no justification for adding to this nervousness by limiting the amount of preparation time.

Teach them a small amount at a time, and review the material that you taught previously. This method does have its limitations. The boys usually know the beginning of their Haftorah because it is reviewed frequently, but they are not as familiar with the end. For this reason, it is important to stress the end as well.

Some boys have problems with self-confidence or an inability to carry a tune. Others with a better voice want to decorate the tunes too much because they feel that they have an exceptionally good voice. Do not begin with the singing, or the trope, but to have the boy read the material slowly and carefully so that he pronounces the words right. Add the melody at a later stage. The trope or cantillations can be added at any time.

the boy should also learn to read slowly during The Real Thing. Too many boys race through their Haftorah in a barely audible voice. A series of dress rehearsals in the shul is useful. Stand in various distant locations and tell him whether he can be heard clearly.