How can I keep my chickens warm in the winter?

Dry and well insulated will go far in keeping chickens safe in the cold. Also, don't let their water freeze. A snack of cracked corn in the afternoon in addition to their regular rations will help them produce more body heat.
I'm not that familiar with silkies but people around here do keep cochins in unheated barns and coops. (not sure how well insulated some of these are.)
You might be able to find out how people near you provide for silkies in the winter. I know some people use heat lamps but I worry about if the electricity goes out and the chickens are used to the heat and not acclimated.
I know some people use it only for an hour or two before chickens go to roost so they are not going to bed in a cold coop. Then when they roost side by side they provide body heat.

 

I currently keep bantam cochins, straight and frizzle, and have had silkies, I've never provided extra heat to any adult chickens, I'm in Wisconsin and we get pretty cold, I've never had a problem. A place out of the elements with good ventilation is a good start. I give mine some corn, cracked and whole, mixed with black oil sunflower seeds, in the morning to get them moving and the afternoon so there's something substantial to digest overnight which generates internal heat. Mine go outside on nice days, I put hay down for them to stand on and they excitedly scratch and pick through it. I give warm water twice a day and occasionally warm oatmeal. Mine do fine, I don't like providing extra heat, I like for my birds to acclimate to the weather in a natural manner.

 

The cold itself isn't all the much of a concern, the biggest issue with silkies is that their feather don't repel water like normal feathers, so as the weather changes and there is cold rain, sleet or rain/snow mix that can spell death to silkies because if and when they get wet their feathers don't insulate and keep them warm... The same can happen with high humidity and condensation, so you need to prove a very dry well ventilated with no drafts and try to keep them out of the rain/sleet mixes or out of puddles/mud on warm winter days...

 

No one ever told my Silkies that the rain, sleet or snow would harm them. In that type of weather, the Silkies are usually the only birds piddling around. I treat them just like my other breeds, they have access to their run in all weather conditions and head back to their coop (which isn't insulated) when they need to.