First of all you need to consider what the right note is for this particular date. Do you like the person you're with? Do you hope to see them again? Are you interested but not quite sure? If you never saw them again in the next 100 years it would still be too soon? How do you feel about this person? Once you can answer this question, you can move onto the next step.
If you have no interest in dating this person again you need to be completely clear about one thing -- you're not interested in them. Don't be polite and mumble half-promises on their suggested follow-up date ideas. You don't need to be antagonistic, or hurtful, but you must be honest otherwise you're creating a problem for yourself.
If they aren't clear that you don't want to date them again, but they are attracted to you, any slight glimmer of hope you give will be enough for them to continue to pursue you. Be kind, but firm -- it's been a lovely night (or "interesting" if you prefer!), but as nice as they are, they aren't the person you're looking for.
If they try to change your mind, don't give, just keep rephrasing the fact that you're not interested in pursuing a relationship with them until they get the message. With some dates you may have to get a forceful tone to your voice before they understand, but most dates won't push the issue once you tell them you're not interested. Don't forget to thank them for the evening however, even though it didn't turn out as you might have planned, you should thank them for the time they spent with you.
If, on the other hand, you are attracted to your date and really want to see them again, then you need to send signals that you would love another date with them. Let your body language reveal that you're interested in them, but make sure that the body language isn't sending out mixed signals about what you're offering!
Get close, but not too close until you want to commit to a more intimate relationship at this point! It's great to flirt as long as you have clearly communicated boundaries about what your date can and can't expect from this date in a sexual sense.
Whether or not your date walks you home, or puts you in a cab, don't wait to see whether they're going to kiss you goodnight. If you want the kiss, go for it. If you don't, don't give them an opportunity to give one to you. Listen to your own personal boundaries on this one, and know when to draw any kiss to a close.
If you want to see your date again, make sure that you have their contact information. Don't rely on them saying "Oh I've got your number, I'll call you." If they say that in response to your request for their cell phone number, proceed with caution. Any date who wants to meet you again will be willing to give you every contact number they have from their home number to their dog's grooming parlor's email address!
Don't leave the date hanging. Know before it ends whether or not there's likely to be another date. If there is, why not try to fix up something before this one ends. Even if it's not an exact date because of needing to check with other responsibilities such as work or family commitments, you could discuss what you'd like to do to -- whether it would be a daytime or evening date for example.
Whether you've had a great time, or not, make sure that you end the date on a positive note. That could be that you walk away happy that they won't call you again, or that you've got their cell phone number in your purse and are meeting them the following day for coffee, but whichever way it ends, make sure that you're not left wishing you'd done something differently.