Mrrzy, I'm not sure if this falls in the rule books but I feel it is important to just Be Yourself.
The best way to ease the conversation all around is if you are relaxed and feeling comfortable. That way being honest, or finding the right words to express what you want to say may come easier.

Amos has some very good points too.
Avoid putting 'a frame' around the person. It helps to be open to anything and avoid placing expectations or assumptions on the person. That way you will avoid setting yourself up for disappointment. If you are not feeling too disappointed but are just aware that this is not the person you wish to continue dating, then it might be easier to find a kinder way to express that.
I am not sure if anyone can give you the perfect 'lines' to say to someone else. It is good to listen to suggestions but in the end, best the words come from you personally.
The manual I try to follow is:
Kindness, Honesty with Sensitivity and to Be Myself.

Bonnie