(Note: I posted this answer to a question about rock BGV recording a few days ago on Quora.com, and I thought I should post it here as well. I added a few things for this post.)
I generally record my own backing vocals. I usually do either 3 part harmonies or 4 parts with the bottom note doubled up an octave. I overdub each part a minimum of 4 times, so there are at least 4 voices on each note. I then evenly distribute each part across the stereo field.
When I am recording other singers doing backgrounds, I typically have people switch back and forth, so one voice is not dominating any specific part. I usually ask for a very forward placement, with extremely bright vowels, especially for rock. Having a male sing high falsetto in unison with a female also produces a great effect.
I lightly compress each track, and I usually bus all the tracks to the same reverb I am applying to my instrumental tracks. As far as EQ, I usually apply a high pass filter, and boost some of the mids and some of the highs, but specific frequency choices depend on what else is going on in the track.
When you have that many tracks, things like ending consonants are not always together, so I often edit out any late endings, and use just a couple of tracks to establish the ending consonants.
One more note on track numbers: If you are limited on tracks, it's a good idea to record all 3 or 4 parts once, then bounce those down to 1 track, and so on. I've always felt like that fills the sound out more than bouncing all the takes of one part to one track.
(Originally posted at Quora)