This one I picked up from an old interview from CCR and I believe the beatles did this aswell. Slow down the recording a bit, then record the backing vocals, then speed it back up to the song's original speed. This will make the backing vocals sound other wordly tight. Because they worked with tape and the pitch would modulate, i don't believe they slowed it down too much. I've done this with my daw and it made it sound really cool. What are some gems you have picked up over the years?