I've found that most drivers give me a little more space on the bike than they do when I'm driving my car. Still, I get a tailgater once in a while.
First, I just do a head check. Sometimes turning around to look at them gets the message across. If they're an attentive driver and they see I'm concerned enough about their location to turn around, they'll often reevaluate their spacing.
If that doesn't work, I will put my left hand down to my side and slowly push back a few times. If they give me some space, I kindly wave a thank you. That usually does the trick, and hopefully educates the driver for the next time they're behind a bike.
If that doesn't work, I follow the MSF guidelines and slow down, increasing my following distance from the vehicle in front of me. The last thing you want to do is make a sudden stop, no matter how far the car behind you is. With drivers being so distracted nowadays, you don't want to chance it. I would never move over and give them an opportunity to pass me. I own that lane, even if I don't need it all. I ride in the left track and protect my lane. If they want to pass me in another lane, that's fine. But if there's more than one lane of travel, it's unlikely anyone is keeping up with me.
***Last week I was riding on a two lane country road from farm country into an urban center. As I was approaching the town, I notice this minivan in my mirrors. It's straddling the center lines like it wants to pass me. There's a car a few seconds in front of me and I'm doing 45-50 MPH (in a 45 zone). I have no idea what they were thinking and as I soon as I had a chance to get away from them (when the road went from two lanes to four) I pinned it. If you see someone being reckless, no matter how many wheels, just get away from them. It doesn't matter if you slow down, speed up, or stop.