Straight Lines: Don't use them. Nature doesn't have straight lines or 90* angles, so use them as little as possible. You can use them in towns, but even then I say try not to: buildings are made with the land in mind, the land isn't made with the buildings in mind. (This goes for making cliffs, tunnels, towns, placing trees, etc..)
Elevation: Levels/layers are important. When's the last time you went for a walk and didn't go up or down at all? Just having small changing in the elevation around the map can help a LOT.
Diversity: A good tileset helps with this, but having different grass, rocks, trees, etc. can help a map be more appealing to the eye. Seeing the same thing gets boring after awhile.
Practice: No one starts off perfect. My first maps were HORRIBLE, but after practicing a lot and seeing what doesn't work, I started figuring out what does work. Just jump in and start experimenting. If you see something you don't like, figure out why you don't like it: is it too straight? too flat? too repetitive? Learning what doesn't work can be as helpful as learning what does work.
Get Feedback: Make some maps, and once you've hit a point where you're ok with people seeing it, post it somewhere and ask for feedback. People here are more than happy to give feedback on maps. You can also look at other threads asking for feedback and see what they did, and what they're being criticized on (I've posted on a few threads giving feedback, so feel free to checkout my posts)