This might be a silly answer, but as an avid reader with a remarkably short attention span, I'll toss in what I've found.
It helps me if I play music and/or stand up over my desk to read. That way I can fidget without actually having an excuse to get up and do something else. I just sort of vibrate and clutch the desk until I can't take it anymore. The music gives me something to flail and twitch to (believe me, it's not dancing) and also helps me not focus on other noises. No matter where you go, there will be sounds. If you're like me, if you don't recognise them, you will wonder where they come from and soon you will realise that you've read two pages concerning the origin of that weird clicky noise from the hallway, or maybe it was that closet....If all you can hear is music, you can't get distracted that way. I tend to listen to the same CD (...or song) a few (or many) times in a row. Some people can't do that. I think it helps keep me from actually paying attention to the lyrics....
If that's not enough, you can also try reading aloud, or reading silently in a funny voice. No, really. 486
There have been many exams that I would not have done so well on if I hadn't made <PERSON> or pirates or my professor in a funny costume read my textbook/dry academic blah to me. It's not the sort of thing you forget quickly.
As for regular fun books, I ascribe to the school of "don't fight it," which is probably useless to you. If you're more interested in remembering what you read than in saving time, just get up and do something else if you begin to lose it or if you start thinking ahead of the story. Don't try to force yourself to concentrate. You'll just get frustrated and eventually give up if you're at all like me. I have to run back and forth across my house like a moth around a light periodically whatever I'm reading. When I actually remember how terribly out of shape I am, I go back and read some more.
If I'm trying to save time, but I can't concentrate for 5 minutes straight, I use a system. Every time I walk/run/gallop/twirl across my house, I have to read a paragraph. If I can't remember a whole paragraph, I have to read the first sentence, and I have to keep rereading it until I make sense of it. By the time I make sense of it, my brain has usually congealed enough to tackle the whole paragraph...
I don't know what's wrong with me, but hopefully something in there will help someone. 486