How can I improve my play in the middlegame?

I'm 1400-1500 rated player, and I'm pretty good in the opening, and when playing players like myself I get a good position out of the opening. However, as the game progresses to the middlegame, I almost always lose my advantage in position, or material. So, what is a good way to improve my play in the middlegame?

Play through master games in the openings that you play. That will show you typical middle game plans. Play through a game once then play it a 2nd time and try understand why a move was made. This has helped me a lot for OTB play.

After reviewing some of your games.  You first need to learn and understand the Opening Principles:

Control the center

Develop minor pieces toward the center

Castle

Connect your rooks

Then...tactics...tactics..tactics...