This guy has a DVD series that is pretty helpful.https://www.sonargpsuniversity.com
When returning to a spot, this may seem trivial, however, remember to have your map zoomed in as far as possible.
Pay attention to where your gps receiver is mounted on your boat. Is it mounted at the console or rear of the boat? That will be the point from which your location is recorded.
Another thing to remember is that depending on the speed you are traveling, you could be quite a bit farther away from the target than you would think. Especially if you're waiting til the object is completely on your screen before you set a way point. Everything on that screen left of the very far right side is history. One way I like to mark points is to use my trail to drive back over the object. I will watch the far right side of the screen, and when I get close to the center (or think I'm in the center) of the brush pile or rock pile I will mark my way point. It also helps to approach the target from the same direction as when you originally marked it when coming back to it at a later date. Typically I mark objects off my sonar and not the DI as well.
Hope this helps somewhat.