Doubling up vocals works well for thickening, but always always always, try to get a good or great vocal take from the beginning, all the effects in the world won't mask the errors you made. Whether it's a cheap radio shack microphone or an expensive one you can still get a good vocal tracks. Here's some tips I just picked up:
During mixing to beef up vocals (remember get a good vocal take first)!!!
1. (up for debate) Use compression on your vocals most of the time they need it for uneven signals and smoothing things out. Just a little bit don't overdo it.
2.Use a noise gate to remove background noise, hissing, etc on the vocal track, don't gate it too much though because some of that noise you still want.
3.Add reverb, just a little to creat spacious sounding vocals, playout with the setting till you like it. Note: Never put reverb on too many tracks it begins to get mushy and smear your track up.
4.EQ to get the vocal tracks sitting right where you want them, experiment around but in a nondestructive way just in case you don't like it you can undo or remove the EQ effect.
5.Encourage the singer, tell them what spots they could improve on and keep the energy flowing. As producers we have to bring out their best!!!!
Hope this helps anybody