I use this sometimes when I have students that have your problem. Make a long downwind turn and when you get to where you think you are lined up with the field fly the plane toward yourself, when your instructer tells you are over the field then switch your concentation on the center of the field. I know it sounds wierd but it works, but remember when you cross the threshhold of the field line up on the center of the field and concentrate on wings level, center of the field, and flaring.
Nine times out of ten you will be in the center when you cross the threshold, but you need to make a long approach to use this method!!