You need alot of "variation tiles" rather then repeating the SAME grass tile over and over again, grass with dirt, grass with a stone, grass with a flower, grass where the blades looks a bit different, Grass bordering on dirt(buffer), etc.
Take a look at warcraft 3 tilesets, it's the same thing, lots of variation, if you are successful no one will be able to tell where your tiles begin OR end (which is of course hard, but hey.)