"All of us are guilty on occasion of fluctuating volume and attack when we sing. Last time I checked, we were all human. Recordings aren't human. There's the rub. You have to be completely consistent in your attack and phrasing for two and a half or three minutes. It's amazing how hard that can be sometimes. :-)"

well, yes, but if you're playing solo you can afford to be a fair bit inconsistent. I mean, consistency of phrasing never bothered the majority of the great bluesmen. And of course there's always compression that can be used to iron out any wild leaps in volume.

It's definitely worth trying to be ultra-consistent if you're going to be overdubbing a harmony vocal. Or if other people are harmonizing with you.

Regarding the vocals generally, one thing you might like to try, Dan, is duplicating the vocal part in your software, and panning each vocal part hard left and right. Then try adding a very small delay (between 20 and 70ms, say) to one of them. The two duplicates have to be panned quite severely, or they will start to phase (being identical). But the ear should hear them as being in the centre.

On headphones, the effect might sound slightly artificial, but on speakers it may well make the vocals sound smooth and thick. You don't "need" to do this - I think the vocals sound good - but it's a trick worth knowing about.