It's simple: Give a server-side benefit.
You can control server-side content, but not client-side.
Allow users to see who is properly authenticated with a unique ID, though don't block authed-nonauthed user games entirely, otherwise regular users will switched to the pirated versions so they can play with the larger crowd.
Rather, release frequent updates that affect the code, and prevent out-of-date versions from plaing at all, that way, the pirate players will always lag behind at least an hour, and the hassle of updating/completely retorrenting every week will be the penalty for getting to play online. Naturally, someone will develop an updater for the pirated version, but unless they are supergeniuses, there will always be at least a 10-minute cut out for all pirated versions before they can release the unofficial patch.
Give neat features such as profiles and profile-actions server-side maybe linking a forum profile to the in-game profile to unlock bonus content such as a user forum section for each game that is read-only for non-purchasers, and allow people to create threads/topics about a server with an in-forum link and status indicator. This will be considered a very neat feature, and allow server mods an easy way to allow playtesting by letting users read everything and join without having to find the server out of the list of hundreds.
Piracy should be encouraged and discouraged at the same time, so that rather than blocking pirated versions entirely and installing invasive DRM, users who can prove themselves to be using a legitimate unmodified game are rewarded.
As a bonus, give out a free demo that can join servers but not create them and doesn't quit after the first hour, and then when an update comes out it will be BETTER than the pirated hacked versions for the first hour(s), and as a bonus, players who cannot afford the game can still play!
If the game doesn't even require a CD to play, but simply blocks more than one player from using a single key online, LAN games will be easier.
To fight pirates, don't use DRM, use server-monitored features!