Be direct and don't lead the person on. If you're on a first date with someone and you don't see yourself in a romantic relationship with this person, tell your date how you feel, especially if he asks. The same guidelines apply if someone is romantically interested in you and you have not dated. Good statements that help you be clear are: "I enjoyed our date and enjoy your company, but I don't see myself seeing you again romantically," or "I am not interested in starting a relationship with anyone right now."
Tell the person that you don't see a connection between the two of you. In any relationship, a dating connection is key in order for the relationship to work. If the connection is not there, let your friend know that a relationship is not going to work out between the two of you, but a friendship will.
Invite other people to hang out. If there's a person who you enjoy spending time with as friends only, invite her to hang out with you and other people. Hanging out as a group and avoiding one-on-one situations indirectly lets her know that you don't see her in a romantic light.
Tell the person about other people you're dating. You don't have to get into specifics, but casually bringing up the date you went on last night will let the person know you don't intend to date him.