There ain't any - unfortunately, there is no way to expor, edit(outside the game) and import maps. I'm waiting on a reply in my post in General discussions, but to no avail. Just ... GL i guess :/

woot

First off, break up your shapes, avoid using straight lines if you want to make it look natural.
there are some functions and generations in the "generate" menu on the pause menu. It has many important and very useful things, like the Scatter function - used for placing blocks onto another blocks randomly - the Median option - which smoothes out the area and the textures (don't use this after you place your ores, as it also applies to ores) - the Terrain generator - which generates terrain randomly based on a few variables. There's also the distort function, which is the complete oposite of the Median.

I recomend you mess arround with the variables and stuff to kind of intrinsically understand the effect instead of studying about it.

There's the Ore generator, which is used to randomly place ores on the map, although I rarely use it, since it makes it look too random, and not natural, I rather place ore veins manually.

Secondly, experiment. Find out what you're best at making and improve it.

Third: vary your textures. making a stone floor? use the scatter function to add craters to it randomly.

Fourth: avoid hard barriers between contrasting textures. use the spray painter to smoothen the transmission to make it look more natural and less lazy.

And lastly, don't be afraid of testing new styles and things, Try anything you think of and make it good.