+1 for Ben's suggestions.
If the SM58 still isn't cutting it - and there is no mic which suits every voice - maybe you should look at large diaphragm condenser mics with less worry for isolation; if the whole thing is a live take, spill might not be a desperate issue, and it can be reduced by building the singer a booth out of boards or mattresses or at worst blankets. Also a shield like the one by SE is very effective:
http://www.seelectronics.com/rf.html .