For my voice I find that rolling off the lows up to the low mids on the double track is the key. That and compressing the hell out of it. I sometimes do two dbl tracks and pan them or if I'm having a tough time getting two good dbl takes I'll just pitch shift the dbl take by 9 cents and then pan it. I've also gotten some good effects by doing a harmony vocal part and mixing it in just a touch. I keep the level so low that you don't even perceive it as a harmony part, it just adds a certain "quality" to the lead vocal.
I've read a *lot* of posts on various forums about vocal doubling and I've worked with at least a half a dozen good singers (myself included) that have had varying experiences with doubling. I really think that it's more of a performance skill than a mixing skill. Some singers can just do it. Most singers need to learn how. They need to practice it. Unfortunately most singers don't. They only ever do it when a producer asks them to "give it a try" and then, regardless of their success (or not) they don't do it again until the next time a producer asks them to try. Guitarist tend to be really good at doubling their tracks. Drummers... NOT!
