An important thing to remember when teaching youngsters to roost on a perch is to place them on it as close together as possible. If they're smashed up against each other on the perch, it simulates the feeling of closeness they are after when they bunch together on the floor. If you happen to be able to identify the most timid of the bunch, the one that refuses to even try staying on the perch, place that chick next to the wall and then place another chick up against it very close, then the next and the next all as close as possible.
Also, by providing a small "play perch" in the brooder, a couple inches off the floor, they learn early to use it. By age six weeks, almost all of my chicks are roosting in the big coop. At that age, it doesn't really matter how high the perch is because they are still light and it's an easy matter to fly up there and down with considerable ease.
As the majority of the crew choose to roost on the perch, the stragglers will be more apt to want to do it, too.