I was wondering how to effectively increase agility. I've already hit the dex cap but my agility seems to be increasing really slowly. does it just go up over time or is there some way to enhance it more effectively? 

Demeter's minigame, walking and successfully hitting an enemy will increase your agility.
Demeter's minigame will raise the agility only for Bell, the other two will raise the agility for everyone in your party.
Walking will give the same quality of Excelia, whereas successfully hitting an enemy will grant higher quality excelia the stronger the enemy is.

You can also increase the % of Excelia gained if you dine at the Hostess of Fertility once a day.