Backing vocals are on of those areas that, like most things, everyone has a different approach to. what i've found really works for me at the moment is recording the lead vocals with an appropriate LDC for the singer and then recording the backing vocals with a ribbon mic. to me, this really helps the backing vocals sit with the lead vocals rather than fighting with them. i'm also a big fan of double tracking backing vocal parts and then panning one slightly left and one slightly right. in terms of eq, compression, reverb, treatment in general in varies wildly depending on the track. about the only thing i do on the majority of backing vocals is put a LCF at about 100hz or maybe slightly higher to, as rhino said, chop out any "mud". otherwise, it's all down to what the track needs. the big thing i've found with using ribbons on backing vocals is that without all the high end on the track it really helps it blend in and sit behind the lead vocals which, alot of the time, is what i aim for.
one other thing that is always important is the performance. as was mentioned in another thread, having the backing vocalist cut out "ess"'s when singing really helps cut down any nastiness. also, experiment with cutting off the end of words at the end of phrases to help lock the vocals together and prevent any overhang of held notes. one of the big perks on logic is the flex time tool can really help lock together vocal tracks so that the notes end at the same time (i normally leave the lead vocal untouched and adjust the backing vocals as/if needed to fit)
In the world of music there's no right or wrong, however there is "tried and tested" and "experimental"