1) If you have a girlfriend/boyfriend/spouse/partner, and the people you are around every day might not automatically know that (e.g. you wear a ring), you should mention that person several times when first getting to know people.
2) If you have neglected to do number 1, and you get the sense that a person is developing a romantic interest in you, be sure to start mentioning that person as soon as you can, in any context at all. If you are directly asked by someone if you have a gf/bf/s/p, you should not laugh at the question: someone wants to know.
3) If you have neglected to do numbers 1 and 2, at the very least, do not flirt with the person who has developed a romantic interest in you.
4) If you have neglected to do numbers 1, 2, and 3, spend a few moments asking yourself why you don't want the romantic interest person to know about your gf/bf/s/p. And then resolve to stop flirting with the romantic interest person, or at least mention your gf/bf/s/p asap.
5) If you can't say the name of the romantic interest person without blushing, or stop staring at the romantic interest person's boobs, again, stop and evaluate.
6) If you haven't managed to do 1, 2, 3, 4, or 5, and the romantic interest person has the courage, bravery and hope in the goodness of humankind to tell you her feelings without asking anything of you in return, if you are going to stick with your gf/bf/s/p, then you should, at very least, tell the romantic interest person a few nice things about her.
7) Do not make up a gf/bf/s/p because you are scared.
8) Oh, and also? Fuck you.