The Wiccan Rede is the closest we have to an ethical guide that all the Craft agrees to. It's supposed to be ancient, or at least based on ancient text; but like so much else, there is no real proof of that, to the best of my knowledge. But it does seem to be very widespread, and it holds a lot of truth and common sense, so I tend to think that it doesn't much matter if it is actually ancient, or if it was written recently!
It comes in a long form, but what you will usually hear is the last couplet,
Of course, in modern English, that would be "As long as it doesn't harm anyone, do what you have the will to do."
Whole books can be written about the implications of this phrase. (In fact, I wrote one!) But I'll to try to summarize the whole thing right here.
To begin with, I think that it is necessary to start by being honest, at least with yourself. Without that, it's way too easy to fool yourself about the rest of it; to say, "I don't think this will harm anyone," when you really know, deep inside, that it's bound to - but it's what you feel like doing right now, and so you want to do it. For instance, a spell to make a specific person fall in love with you is going to harm both you and the other person, because it denies that person's right to make their own decisions about the direction their life is going, and that's really really bad for your karma. Deep inside, you probably know that. But if you really want them, you may decide to ignore that if you aren't being honest with yourself. (By the way, a quick rule of thumb to help yourself decide; if it would be unethical to do something by mundane means, it's unethical to do it by magical means. If you think it would be wrong for you to hold a gun to someone's head to force them to be with you, it's unethical to do the same thing with a spell!)
Then you have to realize what harm is, as opposed to hurt. I define harm as wantonly damaging or destroying someone or something, or as interfering with another's ability to choose their own path. You will probably have your own definition. (I think that the really important part is to think it through, and know what your definitions are.) In those cases where someone is going to get hurt, such as breaking up with a boyfriend/girlfriend, I would say that what we are trying to do here is to minimize the harm. How that is done, of course, depends on the individuals involved. Every case is a special case; that's why a simple list of "do" and "do not" doesn't work very well.
And you have to realize that when you say "harm none," you are included in that statement! After all, you are someone too. Don't harm yourself either. But to get to that point, you have to have a realistic self image, (rare in this society.) In a few words, we must all realize that we are exactly as important as any one other person. It might help here to pretend that you aren't the person in the situation; it's someone else. Then it becomes relatively easy realize, "Sal is really sick, and has to go home now, even if it means breaking up the party." when it might make you feel too selfish to say, "I am really sick, and need to go home. Sorry everyone." Or, if you are a different kind of person, you might realize that it's not fair for everyone to have to go to a certain restaurant when only Fred wants to, and half the group can't afford it. This is still true, even if you are Fred! (I made up these names, by the way. You should too, when you are doing this; I wouldn't recommend using the names of read people.)
There is lots more, which I don't really have the space to go into now, about what love really is, and how (and when) to help people. But the other important thing to realize is what will is.
When the Rede says, "Do as you Will," it's not talking about "do what you want." Will is another thing entirely, a more active principle. We focus our wills, and then things happen. As I say in my book, it's the difference between thinking an apple would sure be good right now, and getting out of the comfy chair walking into the kitchen, and getting one! When we will things, we think about them. We have to, in order to focus. And when you start thinking about things, and thinking through the consequences of your actions to be sure no one (including yourself) will be harmed, and doing all of this honestly, with love, laughter, and wisdom; then you are engaged in Witchy behavior!
For those who are curious, there also exists a much longer set of laws. It's said to have been handed down for several centuries, and I have found it in several different traditions. It's simply called The 161 Laws. Bits of it are very interesting; we especially like the bit about "They must be light as Thistledown." Although we tend to look at ourselves, and say, "Well, maybe two or three thistledown!" We also tend to use the phrase, "Speak not to me of such things, for they frighten me!" all over the place, whenever we encounter something we don't want to talk about.