It's hard to make a high average if there's hills or stop signs on the route.

For example, this is probably one of the fastest rides I did last year (ridewithgps link). We were supposed to have two speed groups, but decided at the last minute to all ride together, and do a mostly flat route. So, to end up with an 18.0 average, a lot of the ride was above 20 mph (click the Speed check box at the red elevation graph at the bottom). I was staying in the draft of the group for most of the way, which helped a lot.

To increase your average speed

Stay in the draft of a fast group.
Avoid stops or hills.
Don't count the warmup miles in your average.

Get more aero. I can ride in the drops easily now that I changed to shallow drop bars. It's a noticeable change.

And/or,
increase your power. See this bike speed calculator for a rough estimate.

For example, take the default settings, but keep Set A at 100 watts and change Set B to 210 watts. The calculated speed is 15.0 mph for 100 watts, and 20.0 mph for 210 watts. You have to more than double your power!
Or compare 150 watts on the hoods=17.6 mph and 150 watts in the drops=19.0 mph. That sounds about right.