Pretty easy things -
-Make buying the game worthwile (Remember the time, where games came in boxes, rather dan dvd-cases and had goodies, and maps, and artbooks, sometimes even actual books and posters and all that stuff with them? You can't pirate those, and they will definately be a selling point. When what you get by buying is nothing but a Dvd with even the handbook on pdf - why bother?
-Seperate Lan-mode, for free download. 99,9% of the acts of piracy that I've encountered, are in the situation, where your sitting on a lan, and want to play a particular game with a cuple of friends. You obviously won't buy ten copies for that. Now if the publisher just allowed Lan-only versions of the game, with no single-player (or maybe a couple of demo-levels) and no online-multiplayer, and gave them away for free, Number one reason for piracy would be eliminated (and sales would even rise, due to more people trying things out, getting interested, buying to play online.
-Treat customers well. If you treat legal customers like their all criminals waiting to happen, you will only make them pirate more.
-More fan-service - free dlc, the means to get to know other players, modding tools, goodies all exclusively for those, who signup in an online-social thingie, that you need to have an original code for. If someone pirates a game, plays single-player then likes it, and buys it to play online, or get dlc - thats fine, a sale is still a sale.
-Make the things cheaper: Why is it, that some games are 30 bucks, some 40 and some 60? That's not due to the 60 buck games being so much more expensive to make, the publishers are just more greedy. And someone who pirates, because he (or she) is a piss-poor student, desperately trying to finance herself, and can't afford to buy all those shiny games she'd like to have because they're so damm expensi... oh sorry, getting a little carried away there. And yes, it's kinda sad, nobody from the industry asks me.