Compression and EQ, primarily.
Grab a copy of
this freebie, the Klanghelm DC1A, throw it on each vocal track and squash to taste. Try it with the "relaxed" switch on and then off; sometimes one way works better than the other.
When you EQ a vocal, the range from 1KHz to 3KHz has the most impact on its "in-yer-face-ness". Softening that range will usually push the vocal back into the mix.
If you still want to push the vocals further back, try a tempo-synced delay. Applying a subtle eighth-note delay to one side and a quarter-note delay to the other side smears the vocal image and makes it less distinct.
Still not far enough back in the mix? Try the Kjaerhus Classic Chorus, especially on backing vocals. Stay to the left side of the "effect" dial - you don't want to actually hear a chorusing effect, just a gentle softening.