Proper break-in and time. As you shoot, the barrel will smooth and copper fouling will become less of a problem. Until it reaches that stage, you will want to work it over to remove the copper frequently. If you have a "rough barrel," it will likely foul regardless of what you do. You can use JBs Bore Paste when the fouling gets particularly bad, but you are probably as well off to use an aggressive cleaner, otherwise.
I had a rifle that fouled very quickley and I thoughly cleaned it with Sweets and then used tubbs final finish bullets to polish the rifling cleaning again every 5 shots. I worked really well for me and would recommend it for any rifle that fouls badly. It is much easier to clean now.