The standard filter which is commonly used on old cars that I am familiar with is the Gano filter:
Gano Auto Coolant Filter Company The nice part is that it can have a clear body which allows you to see what you have trapped. That you way you can judge when to clean it.
I have made a similar filter from a $2 kitchen strainer from Target which are even shaped like the Gano. If you have a clean system there is no need for one. They will restrict flow a bit but I doubt by enough to make much difference UNLESS the screen is filled with crud. Of course, if there is that much crud in the system then it will block the radiator core and cause almost as much problem there.