Sometimes I make 5 copies of the main vocal and pitch shift them up and down. Also I move them around time wise and then punch them in at different places i.e. chorus, bridge etc. Similar to Mr Sayers technique above.
The same for BK vocals. I like to build some options for experimenting.
Of course there is the Nashville or Motown trick buss compression technique for popping the vocal out in front.
Bussing common vocals into their own buss and then blending them back in with a seperate effect, or several, can really create a wall of sound on vocals. A really warm compressor can really tie BKD vocals together.
That's about all I can think of except the singing through a milk carton or other megaphone type item. Also, just repeat singing various vocals builds a nice sound.
Last, you can have the singer/singers do a whisper track, octave track or a variety of intervals so you can have them at your disposal for neat little ear candy at various points in the mix.
MM