Here's a list of ways to stop piracy, but not purchases.
- Go DRM-Free: DRM will be cracked, but pirates often steal solely due to DRM, or wouldn't purchase anyways.
- Make a Club: If people buy a magazine subscription when they buy the game, at no added charge, they will enjoy it more. Do not restrict play for those not in the club, though, but make special content available only to club members. (plug-ins, not multiplayer, should be the incentive)
- Fan Policing: Have fans write letters to distributors of pirate versions of the game, and have your lawyers do so also. Make the fans report back to you about the game, in exchange for an extension of their Club subscription or for credit towards one of your products. (digital only, if you must)
- Go Free-To-Play: I'm no big fan of microtransactions, but go free to play, and nobody will pirate. Then put in microtransactions that are fair and balanced. (allow them to be traded to non-payers also)
- Provide Good Customer Support (and ask for proof of purchase): This needs fan policing to work, but it works. Ensure that the game works well (with money you would have spent on DRM), and make Customer Support a Club bonus (include Basic customer support, and Club customer support, for those whose Club subscription has expired).
- Trust the Customer: If you trust the customer, he will trust you. If he trusts you and is connected to you, he will buy your products.
- Befriend the Customer: Have developer blogs and well answered "Ask the Developer" sessions.
Clarifications:
- DRM-Free: You can have ultra-light (say, limited installs, if unintrusive) DRM, but nothing that hurts the consumer. You do not need a game on more than three computers, and if the person does, they can call for support.
- Make a Club: Do not leave people out of the club out of the fun. However, you can provide neat content for those in the club, and they can enjoy it.
- Fan Policing: Be sure to make this policy well known
- Go Free-To-Play: None of the Disney basic-premium crap. Free-to-Play means no subscription fees, ever. No time expiration, either, that ticks people off. Make it available elsewhere, too.
- Provide Good Customer Support: Employee custom-tweaking of configurations (optimization), and legacy patches for experimental/obsolete equipment are key to making the customer support at the Club level worth paying for.
- Trust the Customer: I'm not saying to stick your head in the sand, just let them know how everything works.
- Befriend the Customer: It's hard, but take a GarageGames style approach. I'd never pirate one of their games, because I have contact (albeit very little, and digitally only) with their employees on the Great Games Experiment site (kinda like social networking for Gamers). Same with Flagship Studio. Give people enough to feel that there are people behind the machine.