When you are out with your rabbit on a harness you need to be constantly scanning and assessing for possible dangers such as predators and dogs and other hidden dangers.As soon as I see a dog I pick Speedy up straight away unless I can see that the dog is on a leash too.If he is going to close to somewhere I am unsure of I put the break on his leash and give a very gentle tug just enough to cause a bit of resistance on the leash and I call him to re-enforce the encouragement to come back to me and change direction.
When taking your rabbit out for walks you normally have to go where they want to go.They will not walk to heel like a dog would,if you want that then don't harness train a rabbit get a dog for that as Rabbits can not be trained to be obedient.Speedy has been going out with me on his harness for a bit over 2 years and he does now follow me for a while but when he decides he doesn't want to go your way he puts the breaks on and will sit dig his feet in and no amount of coaxing will get him to follow.But he does follow me enough for me to distract him from going where I don't want him to go!
If you can get to the point where you can take you rabbit out and are prepared to keep a constant look out for dangers then you and your rabbit will get to enjoy many adventures like I do with Speedy.
For more information its worth checking out Bunny Approved they have lots of great articles on bunny care and this link takes you to their article about harness,they also have a great shop for those of you in the US who have Bunnies.
I hope this helps to make the right informed choice on harness training your rabbit.