You need to try to keep it into quads so that when you convert it to sub d's you dont get the petally things.

Try to keep it into quads then convert, to see whats going on try messing about with a many faced poly cube (made up entirley of quads) by converting that into sub d's, then remove some edges and ver tices to create 5 and 3 sided faces and do the same to see the difference.