I need some advice. Help?

This boy is planning to ask me out next week, but I don't like him in that way. I don't really know how to say no, but still be friends. I like having him as a friend, and I dont want that to end just because I say no. )= AAND I also don't want him to get mad, because his best friend is already pissed at me for no apparant reason. So I don't want them both mad at me. )= How can I say no without without ruining our friendship? Any advice?