- Edited
With harmonies I separate the high and higher and put them either side of the singer
Other considerations are to give your tracks EQ's so as to prevent your sound from muddying the track. For example on Bass you want a low pass filter, below the voice. I normally put all my instruments eventually though a single track and on this I use an EQ with a dip in the section of my vocal range. In my case 80 to about 250hz.
Additionally there is a lot of mud in the lower frequency ranges, boominess from acoustic guitars etc. All instruments need a HPF cutting off everything below 100 hz, except the Kick Drum
I like to visualise the sound as a 3d box(see image)