The "secret", if there is one, to getting tight vocal harmonies and BGV is..... well according to me at least....
Record the best, dead on pitch, properly phrased LEAD vocal track you can.
Now, for the BGV tracks.... be sure the phrasing is dead on with the phrasing in the lead. Pay really close attention to when you start a word, and especially to when, and how, you end it. S's in particular will stand out like lighthouses on a dark night. Nothing says shoddy work like hearing 4 "S"s ending at different times on the last word in a lyric. S's are not the only offenders, but they do tend to be the ones that stand out the best... or worse, depending on how you view it... the main thing is pay attention to the details ... the minute details....
In this song:
http://soundclick.com/share.cfm?id=11962059 I was working with 8 or 10 vocal tracks..... 2 leads, and several BGV and doubling tracks. I used envelopes to control the volume and used that to pull the volume down in a few places where things were not synced perfectly on the "S"s .... The ear is listening for ONE "S" so enveloping the others out is not a big deal. I think if you listen to this on cans, or good studio speakers, you can hear one place where I missed the envelope slightly.
I do believe I pitch corrected every vocal track in this song.... not always dead on by grab those obvious ones and put them in line a bit better.
It's not always needed to pitch correct a BGV. It depends on how loud it is. I always pitch correct the lead and sometimes I do spend the time to PC the BGV. PC on the BGV will also bring a tightness to the vocal performance.
Also, in addition to the phrasing, be sure the inflection on the lead vocal is reflected in the BGV.
That's as good a place to start as any toward getting a solid, tight vocal performance. I can, and often do, easily spend more time on the vocal portion of a project as I do on the entire rest of the project combined. Case in point... the song I used as an example.