You will keep cutting off time for quite a while, but the improvements will be more and more gradual, and will finally stop. Literally, you can only get to be so good, as long as you are a human. threephi gave some great suggestions. I would add three:
First, if you are relatively new cyclist, pay attention to how often you are actually pedalling the bike. I have noticed that newbies often coast almost half the time they are riding. Force yourself to keep spinning the cranks all the time.
Second, work on your cadence. That means train to spin the cranks faster and faster. Aim for rpm of 90 or more, although it may take many months to reach this goal. Ride in lower gears! This might slow you down for a while, but as you rebuild strength while spinning faster, you will be MUCH faster in a few months.
Third, make sure you are getting in enough miles. If your commute is on the short side, get in training rides by either taking the long way home, or riding on your day off.
One other thing that might make you faster--as a last resort--get a faster bike!