First of all, you have to practice. I like to practice on the hard smooth flat dirt of a baseball diamond and on short grass. The tip is to do "the twist". Do NOT try to get perfectly balanced and go perfectly straight. Knowing that you're going to need to make a lot of little adjustments going both forward and backwards, it makes sense to practice "twisting" when idling, going backwards, reversing direction, etc.
Going backwards was really frustrating for me at first ... 6 feet in a straight line, then 10 feet ... progress was slow. But then I started to practice idling and then extending the backwards portion and it really "clicked" for me. Use the smallest wheel that you have, roll toward a fence or wall, come to a stop, push off slowly and start twisting from side to side. See if you can then go to your toes and go forward again, using the twist again.
At some point you HAVE to force yourself to start looking around and looking over your shoulder in BOTH directions. You won't want to, but you NEED to look where you're going to do this safely.
Make up fun exercises that include tight turns in BOTH directions, pedal scrapes, stalls, idling, idling with low still arms, transition from forward to backwards, go over bumps backward. Do backwards loops in both directions, loose figure eights, and then tight figure eights. Stick your arm out and hit a tree or a post and have that spin you around into a backwards transition.
I'm having a blast. I hope you have fun too!