most use braid and top with mono, not visa versa. it gives you the greater capacity of braid and the forgiving stretch and less visibility from mono. anything from half a spool of each to all braid with just a mono leader. think of the mono as a shock absorber. (they actualy sell a bungee like shock absorber for things like dipsy divers in a few different sizes). this way you don`t rip out hooks as often when a hard hitting fish smashes your gear and the stretch helps keep things hooked up when the fish goes balistic.
one thing to remember with braid is to wrap the reels spool with friction or electrical tape. braid slips and spins on the spool if you don`t (and then you have to complety respool to fix it) i do this for mono line too but it`s more important for braid. a dab of crazy glue on braid knots helps a lot.