Like Murraymints said, get some weed-guard (or whatever it's called) and cover it with either bark, pebbles or some other decoration of your choice. It has tiny holes that allow water to drain through it, but will stop weeds breaking through. I think I put 2 (or maybe even 3) layers on my border and since it's been there, I haven't had 1 single weed come through :)
You'll have to cut out holes for any existing plants, but it'll be worth the time and effort in the end ......