One method is to record each part in quadruple. So if you're singing a third and a fifth harmony for example, doubling will fatten but quadruple will double that again. You can copy and slip a track by around 100ms (forward or backward to taste), but the best is a separate performance that is pitch accurate. Sounds more 'live' than digital copying.
Also, whereas a male lead vocal might have a bit of that midrange 'chest' resonance,
Bg's need to sit on top, so sonically you want tracks with a bit more air and breath in them, but not so thin that they have no core. This means playing with the distance from the mike a bit, and when you've got the sonics sounding good by framing the Bg's around the lead vocal at noon, then it's about performance and lots of doubling - all of which you can bounce down to St.2 track afterwards, panning the vocals as you like in the spectrum.