Serve coffee (perhaps decaf.) From what I hear about etiquette, it’s the official This Gathering Is Wrapping Up Now gesture.
No you’re not, you’re just like me! 
Headaches are always convenient and invisible. As you’re ready to move things along, mention your head’s beginning to hurt. When you’re really ready for your guest to leave, just say your headache has gotten worse, so you’re going to take something and lie down. Apologize that they can’t stay longer, but you’r ejust in too much pain to enjoy their company.
Sure, it’s a lie. But it’s a polite little lie that will allow everyone to leave with their dignity intact.
StG
Good for you. But I ask you, honestly, is it really so hard to believe that not everyone has this relationship with their families? I hear this a lot: “Why don’t you love your family? How can you be that way?” And it always irks me that people seem to live in a sort of dream world, thinking every parent-child relationship is perfect! I’m really not pointing fingers at you but I would like to know if you feel this way. Or can you understand - I may like to see my aunts a lot. I love them dearly. I can’t spend more than a few hours with them, though. I went on vacation with them and nearly killed them…the only thing that made it worthwhile was the location.
As for the OP, I am of the type that sets specific times ahead of time. “You can visit until about 5, at which point we have to get ready to go elsewhere.” I don’t think this is rude. Just real life - I have limited time and multiple commitments.
Actually, I was thinking that you sound a lot like me. 
It’s nice to know I’m not the only one who feels this way.
Two words: Gordon and Lightfoot.
When we invite people over, it’s going to be someone we know, and who know our habits and our bedtimes and work times. So no problem.
Drop-ins are different, and I don’t know how to deal with them. These are people who say “Oh, go ahead with what you were doing, I don’t need to be entertained”, or “Can I help you with that?” These are people I like, sometimes they’re lonesome, and it’s hard to cut those visits short if I don’t have a good honest reason. So they stay as long as they want.
But here’s a funny, and maybe it’d work for some people. Daughter and her boyfriend were over on Christmas eve. They left kinda early, and she told me the next day it was because I hadn’t changed the time on my living room clock. They thought it was an hour later than it really was.
So consider that – fix your clocks.
If you’re weird then so am I. That describes me almost perfectly.
You could always pretend you are suffering from Coprolalia…
“twitch, twitch…fuck off”
I start putting them to work.
“Hey, while you’re here, do you mind helping move some furniture?”
That usually does it.
Dangitt! You beat me to the punch wit the Far Side answer. Really though, simply telling guests, “I have enjoyed your visit, but now it is time to go.” Usually works for me.
“Did I mention that my wife recently became an Amway distributor? Honey, go get that presentation ready and I’ll have everyone start filling out the order forms.”
I have been known to say “Okay! It’s been fun! Now get the hell out of my house!”, but it’s only ever to very good friends.
In response to people who don’t understand why you’d need/want to get someone to leave, it’s usually those people who are lonely and really need someone to talk to that won’t leave without a gentle push. Knowing they’re lonely, I want to be able to socialize with them, but at the same time, I need them to respect my boundaries. That includes leaving after a couple hours of visiting. But the people who are lonely tend not to understand this (which may be part of why they don’t get invited lots of places, which causes their loneliness, which worsens their reluctance to leave, and it becomes a cycle) and need some sort of indication that it’s time to go.
Or they’re those annoying people who only talk about themselves and who somehow invite themselves over and you never really know when/how/why you said yes and you’ve been listening to them for two hours straight and you just really want them to get lost.
I tend to do what other people here have mentioned, namely, set a limit at the start or invite people over for dinner before going to some event like a concert or movie. That way they have to leave with you for the event and then after the event you can go your separate ways.
A friend of mine routinely tells me, “I’m kicking you out now,” typically because he’s going to bed. I nearly never leave of my own accord because he loves to tease me about it. I’m generally a pliant, mellow person, and I figure it’s a control thing. He’ll usually fake-whine if I leave, even if he kicks me out within two minutes. (I usually come over in the evening, so I’m going home and to bed, and I stay up later than he does, so it’s not like I’m staying around because I don’t want to.)
I do have to say that it used to really put me off when I first met him, because most people I know would just say “I’m really tired, I need to go to bed soon” or whatever, or I’d excuse myself. Saying “I’m kicking you out” used to really grate me, but it’s not personal with him - and it’s not like I have a reason to stay overnight since I live two apartment buildings over.
As for myself? I generally am the visitor, rather than the visitee for various reasons, but on occasions that I’ve had, I generally just let people stay as long as they like. On the occasion that they wear out their welcome totally, I pretty much put up with it, I guess. If it were really a problem, I’d probably just say I had plans and walk them out, then go to the store or something. I don’t tend to invite people over that I don’t like that much, I guess.
Oh, and I’ll add - I never just drop in. Ever. I always call first and make sure it’s ok. In hundreds of visits to my nearby friend’s house, which at one point were 3-4 times a week at the same time, I’d always call and ask if he was around and if he minded if I stopped by. The only time I’ve ever “just dropped by” was when I lost my phone and could not call ahead, and even then I apologized and said, “Is this a good time? If you’re busy or tired, I’ll go.”
I even tend to call my mom before I come by and she always says the same thing, “Honey, you’re always welcome, you don’t need to call.” Still, I generally do. I find dropping in kinda rude - we’re in an age of cell phones, can’t you call and say “I’m in the neighborhood, do you mind if I stop by” at least first?
Charge them $14.95 per year? 
A married couple I’m friends with tends to be the ‘host’ group whenever the social circle meets. When we have new people, one of them will explain, “You know that point in the evening, when you’re not sure if you should stay or not? Well, we love to have folks over for as long as they want, however… When the time comes that it’s time to leave, we’ll let you know. No uncertain terms. It’s called GTFO time.”
(GTFO = Get The F*ck Out)
And that they do. At the end of any given evening, they’ll say, “It’s GTFO time.” Oddly, if they don’t announce this, they don’t mind if you stay over, even if they go on up to bed (I’ve played MMO’s on their computers while they’ve slept in the past.)