Yeah, that is a key part of it. Depending on the type of server, you can decide how to best keep players. In a hardcore Survival server with PvP and griefing, do NOT do mini-games. That completely ruins the hardcore and PvP aspect, as people will be pre-occupied with the mini-games. In that case, I would suggest hosting PvP tournaments that people can enter to win small prizes, such as temporary donator ranks or in-game items.
For a Creative mode server, building competitions are always a good way to go. It's also not criminal to have mini-games on Creative servers, since building can get old for some players.
In an RP server, you need to pretty much constantly have new things, otherwise, after the players complete the main portion of the RPing, they won't want to play anymore. RP servers are probably one of the hardest servers to run properly.
Vanilla Minecraft: The only people who play on Vanilla servers, generally, are people who like building in Survival and living with the only fear coming from in-game mobs. Mini-games aren't a bad idea, here, but could turn away some players.
All-around (Multi-world, multi-server, multi-purpose): If your server has any of the previous aspects, there are several ways to draw in players.
If you have multiple servers, each with a different style, a competition in one world could translate to another. This would drive players to play on all of your servers, thus giving them new things to entertain themselves with.
Multi-world servers, while not suggested unless you have an amazing host, offer their own positive aspects. You can host a world with Creative, one with Survival, and one with mini-games. With this setup, you can appeal to three versions of the Minecraft community.
Multi-purpose servers would be servers with no clear-cut theme. Basically, people who just decided to start a server without deciding what the people who played on the server were going to do. While it's not the greatest system, it offers a myriad of ways to maintain a playerbase. Mini-games, combat tournaments, and donation ranks are just a few of those ways.
Ultimately, though that may help a little bit, you have to use your own mind to come up with the most efficient ways to maintain a playerbase. It can be hard, but certainly worthwhile.
