How we change what others think, feel, believe and do
If you are sitting, moving towards a standing position signals your desire for conclusion.
Start by leaning forward, not back. Move your bottom to the edge of the seat. Put one foot forward and one back as if you are about to stand up. Perhaps put your hands on your knees as if you're about to push from there. You can do these steadily to show slow movement, or all together to show firm intent.
Hopefully, the other person will see this signal and help you conclude the conversation. If they do not see this signal, continue to stand up, explaining your actions at the same time, for example by saying 'Well, I guess it's time to go now'.
Offer them your hand as a farewell or otherwise initial departure rituals.
A sales person is invited to sit down with a company buyer. When the buyer wants the sales person to leave, she says 'thank you, that's all we need for now' as she stands up and offers him her hand.
A person at a party is sitting chatting with another person. The person wants to talk to others so they start by moving to the edge of the seat and putting one foot forward. The other person sees this and helps them to close the conversation and move on.
Moving as if to stand is a clear signal that you seek to leave. If this fails, then actually standing is completely obvious.
If the other person is sitting, your height will make them uncomfortable so they feel a need to stand or let you go.
If you know that you need to take a certain time or that concluding may be difficult, you can deliberately sit down so you can later stand up, using this very obvious method.