I would suggest you sketch the whole tile set first, or a scene, rather than working on individual tiles like this. You'll never make a 'best dirt', 'best grass' etc. You have to look at these things as parts of an overall whole. I wouldn't even worry about making them tile well, at least, not right away. Do the cliffs on four sides, all the transition tiles, etc, as roughly as you can and then work towards cleaning them up and making them look good once you can see where you need to focus your energy.