Why should people join your server?
How will you set your server apart from all the others on the list? Consistent success is the surest way to maintain interest, but getting to that point can feel like pulling your foot out of three feet of thick swamp-mud without losing your sneaker.
Nobody wants to join an empty server. It's like showing up to a party with nobody there. Established, popular servers tend to fill up and don't stay empty for long. Dedicated players will hop in, and then others will see that it's filling up and will follow suit. If you want people to come in, you should try to fill it up with as many people as possible. Tell your gamer friends to hang out in your server, even if it's just to idle while waiting for other people to show up. If nobody wants to, at least idle by yourself in hopes of attracting people.
The name of your server is very important. Some people try to sex things up by adding colors or symbols to their server name to move it to the top of the list. As there are hundreds of other server administrators trying to do the same thing, that's a waste of time. Think about forming a brand for your server. Running a server - or network of them - is like a business. It should be descriptive of the kind of environment you want to create, but also easy to remember. Ideally, the name should be the same as your web site for the server to make it easier for players to remember. Include relevant information like tic rate, friendly fire status and special rules so players know what they're getting into before they join.
Pick something to differentiate your server that you actually enjoy. If you like a particular mod, go with it. Fiddle with time settings for game rounds. If you don't like campers, switch to short round lengths to increase the pace of play. If you prefer a more cautious pace, enable friendly fire and increase time limits. If you see the majority of the servers on the list going with modified rulesets, consider offering something with more default settings. If you think there's an unmet demand for some kind of gameplay, you can find your niche by offering it.
Ultimately, players will come to you for a sense of community and a reasonably high level of play. The "regulars" will be your baseline. Those people will keep coming to your server as if it were a bar. Learn to recognize the names of people coming in. Say hello to them when they join up. Coordinate with them on voice chat to build a sense of solidarity and to encourage stronger team play. Once you have that solid core built up, you'll be able to expand from there.