Reversing Your Actions:
As I've said before, you may come to a point where you wish to reverse one of your own workings. This can be done several ways, depending on your original working. If you linked the working to something physical, such as a representation of the person, a stone, a sigil, a seal, or anything else represented by a physical object, the easiest way is to destroy or cleanse the object. Obviously, it is difficult to destroy certain things, such as a heat resistant stone, or a sigil made of metal. Because of this, you will want to cleanse it completely of any energies you put into it, then place it somewhere it will lose anything you missed; such as a river, the earth, or under a tree. If you are able to make a high heat bonfire or a cliff, this may work as well. Again, what you decide to do will be entirely dependent on the situation. For instance, burning, burying, drowning, or throwing a representation of a person can be bad, as you may be still able to effect their lives in ways you don't want. In this case, it is best to undo what you did to the representation, cleanse it, and place it to the side somewhere. This will ensure that you no longer have energies attached to it that you no longer want, as well as prevent you from effecting the person unintentionally.
Backfires are also something people will run into at some point, even if you don't believe in the threefold law or any of the rest. Action and reaction is a part of life, and magic doesn't change that fact; it actually is a part of that. Your actions effect you, even when you don't recognize it as such. When you work to help people, you put out into the world that you are a helpful person, and those who you help will appreciate it and possibly help you and others in return. If you put out a grumpy, grouchy demeanor, people will feel more like that around you, simply because your mood effects theirs. Backfires are the same way, in that they are due to a lack of knowledge, a lack of responsibility, or straight fear. Now, I will say this: backfires are not the results of doing "naughty" things. Backfires are simply whatever you do that comes back on you negatively, and this includes when you do "nice" things. The easiest way to reverse a backfire, is never to get it in the first place. Be specific in your workings; do your research and take your time before you go for it. However, if you do find the need to do a reversal, the easiest way is to cleanse.
You will see cleansing repeated several times, as cleansing is highly important to shake off energies you don't want on yourself, on another, or on an object. If you do a working on someone without a physical link to them and you want to undo it, the easiest way is to remove yourself from then energetically; or cleanse them of your energy. I wouldn't recommend this if you cannot feel your energy, the energy of others, and are able to differentiate between the two. Doing so may not be fun for either of you and may make matters worse.
Reversing Others Actions:
Reversing the actions of others can be a bit more fun, but less challenging in a sense. Most of those you would have trouble with breaking the working of, will be too busy doing other things than caring about what is going on with you. Magic is also well involved with willpower and belief; if you believe you can break a working of someone more experienced and you have the will to do so, you may very well be able to.
The easiest way to reverse a working is to have a shield set up specifically for the purpose of bouncing anything cast on you unwillingly back to the sender. It is rather simple, and working with a mirror shield in particular may be helpful while starting out.
Another way to protect yourself, is through disbelief; if you don't believe something can or will hurt you, it will make it harder for it to actually do so. By this I do not mean you simply don't believe in something's ability to hurt you, but rather the lack of any feelings that it can and the knowledge and understanding that it won't. It's rather simple and complex, because it is not easy to wholly disbelieve something.
Another form of easy protection is a good attitude in life. Sounds silly, I admit, but it does make a huge effect. If you are generally a happy, nice person, then it will make it difficult for someone to make you depressed and forlorn. Not letting life, or people, effect your mind state and bring you down will keep them from doing so with energy, because if they can't do it with words or actions, how can they do it with energy? So yes, as strange as it sounds, a positive attitude in life can keep you from needing to use a reversal.
The easiest way to reverse a working that has gotten to you is, ding ding, cleansing. If you cleanse yourself of foreign and possibly negative energies, these energies are no longer a part of your energy, nor are they still attached to it. Taking the time to cleanse yourself daily will be a great preventative measure to take, even if it is just against negative thoughts about you.