If we're inviting people over, the invitation includes the time at which we kick out anyone still there. If anyone were to throw a hissy fit about it at that time, that would be their last invitation here.
I'm also up-front about time limits if I'm arranging to meet someone somewhere else. If I run into someone I know when I'm out and about, I probably do have a time limitation to cite when I want to end the conversation. When all else fails, citing biological needs will work, but not everyone is comfortable with doing that. (Doing the antsy shifting of the body before excusing oneself to go to the restroom sells it, if you don't really need it except as an excuse to end the conversation.)
The body language thing will not work on everyone, as some people just don't grok body language. If you're dealing with someone like that, there's a decent chance that clear verbal communication is the best way to deal with that person, period.