yeah the best way to tell him/her in- person rather than telling him in front of his friends or your friends,texting or e-mailing him about it.
Keep it simple and tell him that you dont want a relationship and why you dont want it....this will make him sad but this time "dont melt" and face it...this is the way to just tell a guy that its over..
The best way is with pure honesty. Don't be brutal, mind you. But I do believe that people deserve to know why, rather than getting some lame excuse and the "I just want to be friends". I also believe that in-person is better than any other way.