I read a paper a while ago that explained a bunch of tricky design that goes into slot machines. The odds of a particular symbol on a reel showing up aren't uniform - they use "virtual reels" behind the scenes. The reels are designed to show a lot of "near misses" where you almost win. They have a lot of partial "wins" that pay less than you put in. They "nudge" reels so it looks like you're getting closer to a win, but the odds don't change. They arrange the reels so you see a lot of "good" symbols, but not in places that actually help you.

I'm not sure this is the paper I read originally, but it explains all this in detail (ignore the dodgy domain name): http://stoppredatorygambling.org/wp-content/uploads/2012/12/...