1. The first rule of casting a spell on someone is this: In order to stay safe, to make sure you're not doing something "evil." Spellwork that comes from a place of light, or positivity, or "The Source" will not harm other people. In your magick, as well as the rest of your life, take care to "harm none" and you'll be much safer. Avoid casting hexes or curses, as the negative energy that you emit from this is bound to come back to you -- according to some traditions, threefold, tenfold, or even more! No one wants that!
2. Don't cast a spell on someone which forces them to do something against their will. This is the case with things such as love or breakup spells. While it is possible that they can work, their actions will not be motivated by their heart. A spell such as this never really creates lasting love, or any lasting feeling, nor will it create the feeling in a sincere and heartfelt way. Instead, what you'll get is a hollow echo of the real thing, and you'll wind up miserable. Trust me, I've seen it happen!
3. Never cast a spell on someone without their permission. Even if you think you're helping that person, it is a violation of someone's personal boundaries for you to determine what you think is best for them. If they come to you and ask, then it's fine! If you ask them ahead of time and they're OK with it, it's also fine. But imposing your will on other people can actually become more harmful than helpful. The one exception to this is when you know that someone is experiencing a real emergency or crisis; just as "implied consent" in an emergency compels medical professionals to treat seriously ill patients, so can spiritual workers imply consent for those experiencing major crises as well.