You don't say where you live, but if it is cold in winter, make sure the roost (not perch, btw...chickens sort of sit on their feet, not wrap their toes around like a budgie) is wide enough so they can get thier whole foot on the roost and then sit on their feet to keep them warm. The wide side of a 2x4 is perfect.
I used to wonder if my girls would ever roost, but eventually one went up one evening and after that, another followed. Soon, they were all lined up like birds on a telephone wire...one thing I thought helped was that I would turn the light off in the coop (if you have one out there) but leave the window open so light from our porch went in; that made the coop itself a bit darker but they could still see to jump (yep, mine jump up about 2 1/2 feet to it) to the roost.
I used to wonder if my girls would ever roost, but eventually one went up one evening and after that, another followed. Soon, they were all lined up like birds on a telephone wire...one thing I thought helped was that I would turn the light off in the coop (if you have one out there) but leave the window open so light from our porch went in; that made the coop itself a bit darker but they could still see to jump (yep, mine jump up about 2 1/2 feet to it) to the roost.