1) Keep it tight. Consider travel time and how frequent certain paths will be traveled. For example, a Pokecenter should be close to the most common entrance/exit of a town, to cut down on travel time when the character inevitably doubles back to heal.
2) Don't over decorate, the players eyes need somewhere to rest. Don't under decorate, every place you can stand on the map should have somewhat memorable/interesting visual variety that sets it apart from another area.
3) Consider walkability. Curved paths may make more sense realistically, but boxy, 90 degree angle paths are easier to naviagate. Make sure the player has enough breathing room while walking (3+ tiles wide paths for example), don't want them to feel cramped.
5) Use as much of the map canvas as reasonably possible. Don't feel you need to fill that bottom right corner of the map? Do something with it anyway, but make it blocked by an HM object to entice the player to come back later in the game. Could be a bonus area that has a special encounter, trainer, item, or berry trees. If you've got the space, why not use it?
General tips would be:
1) Keep it tight. Consider travel time and how frequent certain paths will be traveled. For example, a Pokecenter should be close to the most common entrance/exit of a town, to cut down on travel time when the character inevitably doubles back to heal.