Here’s a technique my students love. We use this when:
(1) we need to drill a repeated section in a song
(2) they haven’t learned the whole song yet, but they really need the completion sense of playing a whole song
(3) we need to keep practicing a song but they’ve run out of the power of concentration they need to make it through the whole song
(4) we’re practicing sight reading, but they feel overwhelmed by the length of the song.

First we talk about what a relay race is. A relay race is one where you run as a team. First one person goes, and when that person finishes, the next person goes, with everyone taking turns until the race is finished. We can play a song that way too as long as we divide it up into sensible sections. (You can get into a discussion about musical form or phrases here, but that isn’t necessary.) The student may play the first line, I play the second line, the student plays the third line, and I finish off the fourth line. Then we may do it again with the parts switched.

This works especially well for songs in an ABAB form. For example, it’s very easy to teach even a beginning student to play the first phrase of Jingle Bells. It falls into a nice five-finger pattern in either C major or G major, and the third phrase is an exact duplicate. They’re usually very excited to finish the song, but they have more trouble remembering (or reading) well enough to finish it off. With relay playing we can have all the satisfaction of finishing a song they know, even though they’ve only learned one phrase!