how can I prevent people from finding and adding me on facebook ?

[Straburreh]

You could slightly alter your name if you wanted. Like in my last name, I dropped one of the letters from it, so that when people type my name in, it doesn't show up because technically it's not properly spelled.
You could also have your profile picture be something other that you (like a cartoon character or a baby picture). That way when someone searches your name, they wont be able to recognize you. The most obvious way people find eachother is by looking up their name and identifying a picture of them. Nothing more embarrassing than friend requesting the wrong person. There's other people in the world with the same name as you, no matter how original you think you are, they would really have to narrow down their search if they were really determined to find you.
Avoid adding specific information about yourself, like where your from, where you work, what school you attend, etc. All of this would only help the person narrow down their options and confirm who you are. There's a privacy setting you can use on Facebook as well, so that no one outside your friends list is allowed to look at your profile, information, anything. Its often used to keep your information secret from the real world, or to avoid anyone stalking you that you do not know about. With the privacy setting on, the only way someone can access your profile is if you accept them via friend request. Even then, they would not be aware if you've recently gone online or not, so if if your ever confronted in person as to why you haven't accepted a friend request, you can always say "I don't really use my Facebook anymore" (in any sense, just lie)
Hope this helps!