Performing magick is serious business, and I would be remiss if I said nothing about ethics. In a nutshell, so long as you harm none you may do as you wish. Just remember that none means none and includes yourself. To those familiar with Wicca, you will recognize this as a rewording of the Wiccan Rede. Those of you new to magick may wonder why these rules apply. To answer that I would like to tell you about a universal law known as "The Law of Three-fold Return." This law says that when you send out positive energy you get that amount of positive energy back multiplied by three. The same applies to negative energy. So, obviously, if you curse someone you are cursing yourself three times over. I have found through my own experience that this law is much more than lip-service. It is real!!
So, what constitutes negative magick? Obviously magick intended to harm someone is negative, but there are also times when magick intended to help can be negative. People have the right to decide whether they want or need help with a situation. If you do not acknowledge this right to their free will you are doing negative magick. So before doing magick on behalf of someone else get their permission first! (Since magick is actually a concentrated form of prayer, asking for permission to pray for them would suffice if you do not want to bring up the "m" word.)
Obviously, since interfering with someone's free will is negative, doing a love spell to force a specific person to fall in love with you would be negative. If you want to do a love spell I strongly suggest that the target be "whoever is right for you." By keeping the target of the spell open to whomever rather than a specific person you are allowing the enrgies of the universe to find the appropriate person for you rather than unfairly--and unethically--targeting a specific person.