If you're working with your own vocalist (also works great with guitars), I suggest double tracking when you want things to be super wide - record two separate passes for each line, and hard pan them left and right. Since they're singing the same notes everything is gonna glue together, but since each pass is going to have minor variations due to the natural imperfections of live performance, they won't phase cancel when you compress to mono.
Alternately playing with reverbs, delays, chorus, or just simple harmonizing can often add extra weight to a vocal track without having to overly process them.