I've had some success overlaying a generic choir patch with my own vocals. Works great for oohs and aahs, but if mixed a few dB under it even adds thickness to articulated words as well.
 
For the female parts I either enlist the aid of an actual female if one is handy, or fake it with Melodyne. I once used the free RoVee pitch-shifter plugin to fake a female part and was surprised at how well it worked. You can hear it on my song "Innocent Lies" (click the link in my sig), in the second verse starting at 1:36. It's not a prominent part but in context I think it's pretty convincing.

I have discussed ADT here a few times and I am looking forward to reading Craig's take on it.

bitflipper
I've had some success overlaying a generic choir patch with my own vocals. Works great for oohs and aahs, but if mixed a few dB under it even adds thickness to articulated words as well.
 
For the female parts I either enlist the aid of an actual female if one is handy, or fake it with Melodyne. I once used the free RoVee pitch-shifter plugin to fake a female part and was surprised at how well it worked. You can hear it on my song "Innocent Lies" (click the link in my sig), in the second verse starting at 1:36. It's not a prominent part but in context I think it's pretty convincing.

That's pretty convincing Dave...when you do this, do you sing it in a high falsetto (say a third below the pitch you want) and then bump it up a third?
Thanks,
Randy

Another cool trick:  
When layering yourself in multiple takes, keep the mic in the same position of the room.  Each layer you record, stand in a different spot. thus giving the effect of one mic pointed at a real group of people who are spread out.

I used to use a product called Clone Ensemble way back in the old days.  I'm not sure it will still work but you could always give it a try.  (www.cloneensemble.com)

Sing the tracks a few times, then use Clone Ensemble (very lightly)!
 

Just had a quick look and it appears promising although as its a 32 bit program I'm going to need a bitbridge to use it on a 64 bit OS (Sigh....... Something else to learn !)