What's the best way to end a conversation?

For both phone and text/app messages, in person is a little different. I'll be talking or texting back and forth with someone and they keep just bringing up new topics or asking questions and after a while I'd like to end it but have trouble figuring out how to gracefully conclude the chat. I can sometimes fib a little to get out of it (ie "I need to leave soon to go and do blah-blah") but that's kinda dishonest and doesn't always work. It also feels kind of rude to end a text chat that way.

How do you politely end a conversation?