One of the great things about a unicycle is its ability to go backwards, so we recommend taking full advantage of it. It's very amusing to go careening straight toward some innocent person, then suddenly reverse at the last second! Besides, it's inevitable that you will be asked, "Can you ride backward?" and it's nice to be able to say, "of course."
But how does one learn how to go backwards? Some say it's like learning how to ride all over again. Personally, we didn't think it was that bad, because think of all the things you have going for you: you can already mount, and you have a pretty good sense of balance, and you have more courage now.
Basically, you can use all the techniques you used for learning how to go forward. These may include holding onto a fence as you ride, holding onto a person, or riding backwards between two other unicyclists going forwards. Another way that we found worked quite well is simply to push off from a person or fence. Or, better yet, mount and ride backwards before you even go forwards. Put the pedal for your strong foot at a little lower than horizontal, then step down on it and you'll start going backwards.
Riding in a straight line and steering will come with time. When you get better, try turning corners and riding in backwards figure eights. :)