Ohyus~ The tips posted above by Tidey and Sura are fantastic, and all I have left to say (that I can think of at the moment), is to try to tell a story with your maps.
I know that some of my most favorite maps have been based around a central idea, or story, and the landscape and some of the formations (be it cliffsides, or a small, sunlit clearing in the middle of a dense forest) hold some kind of significance or history to me, or my characters.
For example, if you've got an RP or some story in mind that has a past where some battle was fought; see if you can build a map around it. Perhaps in the heightmap it could be in the middle of a ravaged valley with stark cliff faces, or fallen trees and a lonesome waterfall that tumbles into a pond, edged with black sand and stones, perhaps some of those stones representing those fallen.
I 'unno. Have the landscape tell a story to those who see it.
Detail is always key, and...
And and and. Look at what other map makers have created. See if you can take inspiration from their work. Look at real landscapes, or try to recreate even some pretty area around your house, or something like that.
Inspiration can come from anywhere; You've just gotta look around and see what comes to mind. c: I'm sure you come up with something great, floof~