Think over what you want to say in advance, and write down the two or three most important points you want to make.

Be direct.

Let them know directly that there's something you'd like to discuss. Be sure you have their full attention and be direct in your language. Say, "There's something important I want to talk to you about" instead of "Hey, when you have a moment I'd like to talk." 

Pick a good time to talk.

Try to approach them at a time when you know they'll be less busy and more able to focus on you. You may even want to ask if they could talk at a particular time so that you know you have their attention. 

Write it down first.

Some people find it easier to put their ideas into a letter. Let the other person read it and then have your discussion. 

Disagree without disrespect.

Parents are only human, and they can feel offended when their views are challenged. Using respectful language and behavior is important. Resist the temptation to use sarcasm, yell, or put down your parents and you'll have a much better chance of getting what you want.