Sight-reading?

How do you do it? What should I do to force myself to sight-read? Mind you, I don't have and can't afford a piano teacher.

I have a couple of books I am using to progress and it's full of short exercise pieces (something like, 8 to 32 measures or so). So there is a lot of new tunes I learn all the time, and I need to read them to learn them it's even too much to memorize it all, I play them from the book.
Some are easier, some trickier and take a lot of time to learn to play through. Basically every week I learn a couple of these things and I improve a couple others I learnt earlier, so I end up reading a lot.

I don't really practice sight reading but my teacher makes me read sometimes, every other week for 5 min or so and on those occasions I see that I am ok-ish in sight reading even tho I don't specifically train for it just use it as a means of learning new tunes.