Practise is the first step I think. (But the biggest one- the others are irrelevant if you don't sound good.)

Knowing people has got to be big as well, (or knowing good agents/managers etc... who know people.

Individuality. (In sound, appearance, name, everything really. That makes you memorable and recognisable.)

Too a certain extent (and oddly in contradiction to the last one) being part of a general trend.

A certain ammount of business acumen/general craftyness.

A lot of these would bleed into each other anyway, and they can all be luck influenced as well.