I know this may sound discouraging but realistically, if you refer to 'commercial' tracks (and I suppose you mean the big budget major productions) you have to keep in mind at least 3 things:
1 The performers are usually technically top notch and can sing the same phrase several times in a row with minimal differences between the takes. (I had the honour to record singers who can create a flanging effect on the lead with a double - which means the double is virtually identical!) Any well trained vocalist should be able to do proper doubles.
2 If there's the need for doubles the production will not pass on to mixing until they are perfectly tracked.
3 The cost of the recording equipment of larger commercial studios exceeds by far anything than most of us can ever afford privately. When used properly, top class equipment will reduce the need for DSP and all sorts of digital tricks to a minimum. (Well, somebody will always come up with the argument 'hey I've got 24bit/96khz too'. The question then is 'what are these 24 bits filled with?')
Back to 'modest' reality and your question. I get the best results, especially with doubles on female vox, when I do a sharp shelving low cut at around 3 khz until the doubled vox sound really thin and breathy. When mixed under the lead vox it gives this nice 'gloss' on the vox. Both have to be panned in the same position btw. Like that it feels like one voice but with that little extra. Once again, keep in mind that the double has to be performed in PERFECT SYNC with the lead. And trust me, it IS possible. I know at least 5 singers who can do it within one recording session and quite effortlessly. We usually try to get each usable line twice - also each individual chorus phrase - and treat them as I described before. Like that you get x takes with x doubles but all sound like they're one when they're in the same pan position.
BTW, lots of compression on the vox make them perhaps stand out more but also make them sound 'smaller'.
easy
B#