Try to get into the habit of placing elevation first judging by the type of landscape you want, adding any cliffs and water in the process. I'll usually have a large area of map done first like this before moving onto anything else - almost as if I've used a height map to layout the earth.
I find it easier this way to make more realistic maps as you arn't building unrealistic elevation around houses, your building houses depending on the landscape. Your placing roads where flatter ground is and building villages in spots near water and defence, you're elevation isn't so random and you're cliffs arnt square.
Well, that's the way I see it. Then you can just alter the elevation slightly if need be as you go and start terrain mixing nicely. I usually do all my elevation/cliffs etc first, then my dirt 3, dirt 2 and dirt 1 around the hill edges and cliff basins, using grass 3 to help blend it too. I then start to manually place trees/rocks and then other objects until it develops into a nice looking scene, terrain mixing along the way if necessary.
Similar concept with towns and villages, placing roads, farm and buildings around realistic points in the landscape.
[This message has been edited by Tetsuo Shima (edited 08-13-2011 @ 12:11 PM).]