Low wattage heat lamps, like say 75 W each placed overhead securely where they cannot be knocked down into litter to cause fire. Reflectors a must.Secure mounting a must. If overhead, need to be no less than 18" above roost pole. If cannot do that due to size restrictions, place one each in all 4 corners at ceiling. Alternative is an electric space heater set in center of coop and set on med to low. Helpful to throw bales of hay or straw on roof of coop and pile up on North wall outside too. Leaves, pine boughs, etc to trap heat. Can improve/refine in warm weather.
Do be certain to predator-proof your coop! They are patient and very canny creatures, and they are more hungry in wintertime.
On predator issue, I forgot to say that chicken wire is no good against coons, dogs, coyotes, badgers, groundhogs, etc. Welded wire and chain link has holes too big and coons will reach thru and decapitate your chooks. Hardware cloth mesh is best.
I put a big bracket on the inside of the coop and have a trouble light hanging from it. I don't know how much heat it really puts off but I hope it's enough. I also use about 4-6 inches of shavings on the floor. I have cold hearty birds too.
Yes, what ranchhand said--we really need to know how many chickens, size of coop, and expected minimum temps. Very few places in the USA require added heat, and it is usually best not to. It is more important to insulate in very cold areas, and provide good ventilation to keep the moisture level down. I'll try and find some links...