Well, one of the first rules of recording is that the recorded sound is only as good as the signal chain....
The Actual Performance > The Room > The Mic > The Cable > The Preamp > The A/D Converter > ect ect.
Assuming that you want the sound of a dynamic mic, I'd first ask about your performance (no offense

). Often when a vocalist is recording over finished tracks, they don't put the same energy in as a live show b/c they don't have the feeling.
As far as mic positioning....With cardiod mics you have a proximity effect. Are you 'eating the mic' during live shows so that you can hear yourself over the stage volume? Where do you have it placed when recording and can hear yourself better? When I'm going for a Dynamic sound, I usually place the mic fairly close to the singer, but at nose level facing down and have the singer sing straight ahead. This off-axis approach usually helps to keep some proximity low end, but helps to eliminate the "P popping" that occurs when close miking.
If you're still not finding the sound you're looking for...consider a tube preamp or even recording with a compressor on.
I've also found that singers usually get more agressive when singing with themselves. Record a scratch track, and then sing with it while recording a second track. you may be surprised.
Justin