Hello ! There's several way to improve in mapping :
First : Copy/Adapt/Rework. You can look at maps made with similar tileset (Or not ! Adapt beautifull maps to your tileset and standard ) and see if you find them pretty, and why. Then try to "copy" them but modify location, road, etc... It will come naturally after a while, a lot of do and redo will help.
Second : Questions. Ask yourself a lot of questions according to the type of map, imagine a forest. Is there some wildlife ? How big the tree should be ? Why is there a road here ? Do people come for woodcuting ? So i could add cuted tree trunk. Etc...
If you do a city/village you can ask for other questions : Where do people get food ? How and where do they buy something ? What can they do for a living ? Why is there a road leading to the castle of the evil lord (That's stupid, or perhaps he stole the castle...) etc...
Third : A good tileset. Even if you have very good mapping skill, if your tileset doesn't give you enough materials for the kind of world you want to do... well, it's not gonna be good.
Fourth: Details. If you pay attention to details it will give a lot of life to your maps. Littles animation like wind in the tree, animated water, bubble in lava, critters, flowers, lighting, shadow, etc...
Five : Does it look natural ? Avoid pure symmetry and nonsense mapping shortcut. Even if your world has magic or flying pig, it also have rules, define your rules and stick to it, if you do something a way, it would be illogical to do it differently later, except if there's a good reason !
Hope it can help.