OK there is a right way and wrong way to swing your metal detector. If you are doing it incorrectly your chances of finding anything good gets less and less.
One of the biggest mistakes you can make is not swinging correctly.
Before you start swinging your metal detector you should make sure the shaft length of your metal detector is set to the correct length. The coil of your metal detector should be parallel to the ground and just out in front of you.
Once your metal detector it set to a comfortable length you can begin your swing, with one thing in mind. When you are going through your sweep try to stay focused on the position of your search coil. It should remain as level as possible with the earth has you sweep.
Remember you are not playing Golf
Keeping the coil of your metal detector perfectly parallel at all times does take practice. So you are going to have to pay close attention to the sweep or swing you make for a little while. You should keep in mind that arching your coil when sweeping is going to make you lose depth with your metal detector.
During an arched sweep, the only time the search coil of the metal detector is ever level with the ground and is at an ideal distance from the ground is at the center of the sweep. When you are at either the far right and far left of the swing chances are your metal detector will be missing surface finds.
Top Image from: Detecting365 and OzarksDetector