When unsure of how your trailer will react when you go backwards; look at the bottom of the steering wheel. The direction the bottom of the wheel is going is the direction your trailer will go.
When making big adjustments while backing into places turn your wheels while stopped rather than while your rig is moving. You can get the trailer to turn in less distance if you turn your wheels before you start moving,
There is an art to lining up and positioning your vehicle and trailer before you put it in reverse. Pull up closer to one side, turn so the trailer starts to point to your parking spot, turn your tow vehicle sharply during the last few feet so it is ready to direct the trailer before you ever shift out of drive.
Use walkie talkies, speakers on cell phones, bluetooth is the car for your spotter to easily communicate with you. Even if they cant give you perfect directions, at least they can tell you to stop before you run into anything.