break it to them gently, its never any fun to be on the receiving end of a break up..and do it calmly, throwing blame will only start a fight.
simply tell them that your really sorry...that youve had some wonderful times, but you feel that your relationship has run its course....and that you enjoyed the time you had, but you dont see your relationship going anywhere.
if they cry, try to comfort them..but dont cave in and change your mind because of tears, because then your staying for the wrong reasons and you will then be even MORE miserable.
and its ok to stay friends afterwards, but when you fisrt break up...its good to not talk to one another for a few months..just to ensure its a clean breakup.