This 5 tips are very general, don't expect any magical formulae. They are based on my 5-year experience with the free spanish UO shard in which I help and do some administration tasks.
- Have a Website or at least a blog. Writing news, announcing new features/patches, changes and important subjects is vital.
- Provide official forums. While this needs aditional human resources (at least an administrator), part of it can be handled by loyal and friendly players (forum moderators). If not present, the players will create themselves forums, but if you provide them, you can have all the player base in just one place.
- Make contests, polls, and other participative events. Players can actually help you designing new logos or graphics, levels, or simply ideas for new game objects. Reward them with public recognition, a decorative in-game object, a t-shirt...
- Allow them access to additional tools or directly provide them. Why not creating a Wiki for the players to fill with game basics, lore and other info they find useful? If most of players use a tool or third-party software, place a link in your website to download it.
- Advertise your game, and promote advertising of your game by your players. Run a contest to design signatures promoting the game. Search for gaming tops or online game directories and add yours. Promote it in gaming portals. Send news to specialized game new sites.
While our player base has a quite steady graph (the player base grows very slowly), we have players who have been since the shard (server) was created, and some others keep coming back over and over. We're starting now to adquire new players who didn't even know the game (in the past it was more like word-of-mouth).
It's not easy to keep luring people to play a game 9 years old :)