Standard D&D is pretty high magic by default. You could probably just run a standard game, just make magic more obvious. Magical street lamps that automatically come on at night. Every squad of the city watch includes both a mage, and a cleric from some deity of law and order. Have a mage academy and a street of the gods (where all the temples are) in every major town. Have bars use cantrips to provide ambient music and atmosphere, etc.
If you want to make it more high powered, give NPCs levels in PC classes instead of NPC classes. Allow NPCs to be higher level in general. For example, an elite squad of city guardsmen in a major city might be 10th level or higher.
For inspiration, I suggest looking to the Forgotten Realms (and the Netheril era in particular) and Eberron.
One idea that Netheril has that really evokes a high magic sense, is the concept of pseudo-magic items. Basically, each city has an invisible field of magical energy around it (referred to as a "mythal" in the Realms). Within that field, special pseudo-magic items function normally, but taken outside the field (even to another city) and the items cease functioning. This is a good way of arming people inside a city, like guardsmen and the like, with powerful magic arms and armor, yet if the PCs get their hands on it, it doesn't function outside the city. Furthermore, such magical items may have the equivalent of an arcane tracking device, so when stolen, the city mages can track the exact location. Of course, you'll want to provide plenty of regular magic items as well. But this allows you to give the NPCs a little more fire power without worrying too much about the players getting too powerful if they get their hands on it.