Create a focal point on the map. It can be a city, a mountain, a landmark or something symbolic that defines the entire map. It has to be something that everyone living in the mythical world seeks. This will form the foundation of a land.
Create different areas offering something unique in terms of resources. Create a mining town, an area dominated by mythical creatures, an area where the port holds the most importance, an area built to serve the aristocrats and so on. This will fill up your map and give it life.
Different areas should have different climates. This will add an extra layer of depth and diversification to your map.
Do not create empty areas or cities and towns for the sake of it. This is counterproductive. Keep only as many areas as you can fill with quality content. By doing this your map will look good and seem to be accurate.
Create memorable landmarks such hills and valleys or a castle on the mountains or something that will resonate in the viewers mind.
Do not use straight lines drawing the map, even when it comes to roads. Have them curving through the hills or something of the like. Be sure to give it an authentic feel by making all the roads look worn out and regularly used.