How much fluctuation are you experiencing? Most tanks will fluctuate a few tenths over the course of the day, because photosynthesis raises the pH.
One of the keys to a stable pH is your alkalinity. The alkalinity of a solution is the ability to resist pH change when acid is added. Your alkalinity should be between 2.5 and 4.5 mEq/l. In general, alkalinity tends to drop as water quality degrades.
Depending on the tank setup and biological load, this can be maintained by regular water changes, or by addition of buffers like baking soda. If you are keeping corals or clams, the alkalinity and calcium need to be monitored and regulated together.
Anyway, it would help a bit to know how much your pH is fluctuating, your alkalinity, and how often you are doing water changes.