Well, there are specific cues that can be given which indicate non-specific information; descending tones in a sentence tend towards definitive announcements and represent an appearance of authority, while ascending tones are inducements of affirmation or agreement. They are both useful in their context...but when you need to communicate the end of your involvement in a conversation, you may find it less than useful to seek consensus (which is what you would communicate with the ascending tone); instead you may wish to firmly communicate your boundary or limit (which you are more likely to do with a descending tone).
Blueberry's suggestions are methods of breaking rapport, which is usually established by full-body mirroring in most people (mirroring posture, hand position, leg position, head tilt etc); rapport is a method of gaining comfort with someone you are dealing with and people in rapport are usually reluctant to leave it. Making a deliberate choice to do so can be an important step in easing oneself out of a conversation.
However, there are people out there who associate breaking with rapport with rejection of sorts; the reasons vary greatly and it usually boils down to a lack of clear boundaries between involvement in one's life and involvement in another's and where the line of separation is supposed to lie in their model of the world. At times like this, clearly stating your stance and your priorities (I have enjoyed spending time with you; I have a lot going on and need to attend these other things) helps clear some of this up (or at least gives them something to work with and induce a learning in them if you're lucky) as does declaring when you expect to see them next as you go. Just make sure you are congruently communicating to the other person as you do so; mixed signals, as always, confuse things.
While the song helps to remember the specific order, in order, of the alphabet, I just went ahead and found patterns in the alphabet. Can you remember the vowels? What does the alphabet look like without them? What letters are between a and e? e and i? Which letter is in the middle of the alphabet? Knowing those answers (and others) helps break the entire string up into chunks that you can manage easily and cross reference unconsciously with the entire song memorized so you can recall the relevant information quickly and easily. The practice also familiarizes oneself with the alphabet itself overall and other connections and patterns will be recognized in an out-of-conscious manner.
Thank them for their time, sincerely, making sure the beginning of the statement acknowledges the value of the current thread of thought ("that's absolutely fascinating...and thank you for sharing that with me") and make sure your tone of voice descends at the end of the sentence; if they respond with confusion at this abrupt ending (it may appear so to them) let them know why you must go now or soon.
If your reason is impolite ("you're a boring jackass") you may wish to omit what you specifically think of them (the reasons why you think they are a jackass may have less to do with them and more to do with you and how you see the world subjectively, it's something that needs to be checked out at some point) and simply indicate that you are in disagreement with them and that you lack the time and energy to properly present your position and that you may or may not get back to them later.
Works 5/6 of the time.
Oh wow. I've seen this issue from about a half-dozen different perspectives.
Starting with my dad who moved from Engineer to Manager at the company he worked at. He hated it until he learned a few basic skills about dealing with people's perspectives' and understanding of the world and how different perspectives on a situation will necessarily generate different approaches, assumptions and beliefs that will filter information received by them as individuals until some critical information both matches the current belief and induces a transition to other belief systems.
Anyways.
Managing people is mostly about making sure that they are functional and have their needs taken care of; a lot of factors can impact job performance in any field that have little to do with the job itself. When you become manager, take the time to find out where they're at in their life and what can stress them out or what they are looking forward to. See what tools or policies you have as a manager to help with that, or what strategies you have employed in the past in regards to workplace policies that can smooth things out for them. This step is a maintenance step and requires a routine checking in on.
Next is inducing excellence in them. This is partly understanding what they are good at and giving them tasks suited to them, and working with them to overcome obstacles they come across and to arrange to have them learn new skills to enable them to perform better. This is a competence awareness process that allows you to break a project down into the largest chunks possible according to your subordinate's skills.
Other than that, make sure your perspective is clear of bias as much as possible. You're dealing with human beings who are, for the most part, trying to get by to the best of their ability in a competitive and stressful environment. Each of them has arrived to where they are by making the best possible choices they could- there is every chance that if you had lived there life, you may very well have made identical if not similiar choices.