is just tell your team you know know how to play x role.
they get so mad at you and tell you to get out of ranked if you only know how to play 1 role. But they still usually want to win so they let you play it. it is of course not foolproof 100% but it works better than any other way i tried
What is the best way then, if TC's method isn't the best?
"ADC pref. Please anything but Jungle. Lolking my IGN for ADC stats."
Gets me ADC 90% percent of the time I ask for it. And I have NEVER been forced into jungle.
And I rarely have people actually check my stats, though I wouldn't be negatively affected if they did. Not providing a link usually makes people too lazy.
You don't have to be a douche to say what role you want to do. You CAN exempt yourself from a role if you don't think you're competent at it, but to imply that "I CAN ONLY PLAY THIS OR I'LL FEED" is something that no team wants to deal with. (Plus to be honest, if you don't have the mechanics to play those other roles, that probably means you suck overall.)
EDIT: Just for clarification, I don't always ask for ADC. And I don't always exempt jungle. I play all roles, but I get into moods when I queue up where I want to play something specific. This is an okay attitude. As long as you don't be annoyed or upset when you don't actually get what you want.