An experienced spotter can be a big help. My wife tries to help, but usually her advice isn't much help because her only experience is with bumper hitch trailers. I'll be at close to 90 degrees with the wheels cut all the way trying to straighten and she'll tell me to take the back end of the trailer the other way. That isn't going to happen until the truck is straight, so it really isn't much help. I keep asking her to just watch the passenger side of the trailer and tell me if it is going to hit anything and also to help me judge if the trailer is straight when I ask. Our teamwork is a work in progress.
The hardest part for me is figuring out where to make my initial cut. I have a pair of orange gloves in the truck, and I'll drop one out the window when I cut the first time. If (when) I have to try again, I can use that glove as a reference to how to change where I start my turn for the next attempt.
Don't be afraid to get out and look. Beats heck out of running a site marker down a dually fender :E