If you have mules, you know that they are pretty smart creatures. You also know that they are very independent. They may not want to stay where you want them to. To that end, you need to have some pretty strong fencing, and you may need to come up with some tricks of your own to keep the mules where they belong.
One thing you can do is to install an electric fence. An electric fence isn't a cruel thing. The fences have just enough zip in them to remind the mule that they shouldn't go there. If you are worried about how an electric fence looks, you can combine it with something else. For example, you can put an electric fence just inside of a board fence. That way you get the pretty fence that you want, while still keeping your mule contained. Another benefit is that the mule may not be able to reach enough of the fence to chew on it. You can also turn off the electric fence any time you want.
Planks on the Inside
If you decide that you want to go with just the wood fence without any other option, you will need to make sure that the planks are put on the inside of the post instead of the outside. That's because the mule can lean on the fence and use their weight to push out the planks. Once a plank has been pushed out once, it will create a weak spot because the holes where the nails or screws were will be enlarged, meaning anything else that goes in there will be loose. The mules will remember where that area was, and they will push out the boards again so that they can escape again.
Make Sure of the Height
You don't want to your mule to figure out that they can jump over the fence. The best way to do that is to make it just a little higher than you think it needs to be. It's always better to have the fence a little too high rather than a little too low. If you are going with a larger mule breed, you will want to make sure that everything is high enough. The mules should be smart enough to realize that they can't jump, but if you have a really stubborn one, they might try once or twice.
If you are getting a mule, you want to make sure that you have the right fencing. You can always talk to a fencing company, like http://www.rapsfence.com, that specializes in working with fencing for animals.