When you are with that person, just try to act normally or like how you would with any other friend. You can flirt but don't try to make it obvious. Also try to get into the mentality that when you are with them, you are strictly as friends and that nothing more will happen. That way your actions will reflect that and you will definitely remain as just friends.
If you feel like you just can't do that, then try listing their flaws. Even though it sounds pretty awful, it's a good way to get over your feelings if you wish to say friends. It also helps if you find someone else to like too.
Note that this answer is if you just want to stay friends. I'm not saying that you guys have to only stay strictly as friends. If you actually want to pursue them, then go for it.