1. When I first started to learn this someone told me to head straight for a wall or fence and hit it to start the backwards motion.

2. I found chain link fences to be better for this since they had a little bit of spring to them when you hit em.

3. Although it may seem counter-intuitive going at the fence with some speed helps, I'm not saying book it at the fence but If you have too little speed you wont have the same amount of control. Kinda hard to explain but it's kinda like riding with no handlebars the more committed your are to going backwards and the more comfortable you are the easier it will be to ride backwards.

4. Some claim that clips will help, I haven't noticed any benefit from doing so but some people swear on this.
Hope all this helps.