I have tried several methods, with no clear winner. Initially, I used a bucket with poultry cups and a bucket heater, but the valves freeze when it gets to about 25 degrees. This is a simple system for me, because all I have to do is carry out a bucket of warm water to refill. I also have a plastic 3 gallon heated unit. (the plastic bottom is the heating unit). This is nice because it lasts for 2-3 days with our flock of 10. However refilling it is a major pain and have more than once flooded the coop when I turn it over and it is not correctly lined up. (Trained myself to turn it over when I'm not in the coop, but had a steep learning curve!) Also hate this because my daughter cannot manage this on her own. Technically, they are "her" chickens. Third thing I tried is a square dog bowl. Adapted it by adding a one gallon milk container with lid filled with water and a cut slit into it. This does two things: keeps birds from roosting on the edges (or falling in) and it lets water drain into bowl as the chickens drink, lasts 2 days longer, and keeps bowl cleaner.