One way to look at it also might be from your own perspective.....how is the best way that you take a guy that you are interested who is not that interested in you letting you down?
Honestly is always the best because when we try to say things in other ways they sometimes get misunderstood or misinterpreted. Always good to be honest, straight forward but let them know that there is just not enough in common for a relationship to work..
A lot better than using the line....."it's not you it's me".
However like S4ndm4n said...if the guy refuses to get it or is a real lech sometimes you have to be aggressive. Sometimes the baseball bad (or rolling pin) becomes necessary depending on the person you are dealing with.
I had a guy that I just wasn't into for many good reasons like controlling, but I enjoyed riding horses with him & an occasional dinner & movie but he wanted me to commit to specific dated 6 months in advance just to go out to dinner

I politely told him that I NEVER plan my life that far in advance & I definitely don't even usually plan what I'm doing the next day until it gets there so there was no way I was going to do that. I was seriously getting the feeling of how controlling he actually was & started just not being available to do things with him. I'm glad that I caught the red flag because I knew him through 2 marriages until he died of cancer a few months before my mother did & his last wife was so relieved to be free of him in the end as he wouldn't even let her go out of the house or do any of the things that were her interests & his first wife just up & left without saying anything. Looking back I realize that she valued her freedom more than she valued the things from the marriage.
Good luck with this.....I'm sure you will handle it in the most respectful & kind way that is needed.