If you're unfamiliar with the Minecraft forums, they're a place where Minecraft players gather to share information and post about anything relating toMinecraft. Plenty of people visit Minecraft Forums and if you haven't joined any yet, you should look into it. Gaining respect in a Minecraft forum is a great way to meet new Minecraft people with the same interests as you and potentially bring them to your Minecraft server. Here are a few forums you should join and stay active on:
Be sure to put your Minecraft Server Info in your Signature feature.
When posting make sure to follow the rules of the Minecraft forum and post in the right category. Nobody wants to see your Minecraft Server promotion in the mods section and look at it as shameless advertising.
If you have a video making skill set then you can put those skills to the test by making a video to advertise your Minecraft server. There are many videos out there already talking about other Minecraft servers. You just have to make one regarding why people should play your Minecraft server and what's so great about it.
You don't have to do this by yourself and can even ask someone else to do it for you. That way if anyone is searching Youtube for a good server to play, yours will pop up!
Minecraft Video ideas include a legitimate advertisement or you could get Minecraft players to make testimonial videos talking about how great your Minecraft server is. Both ideas work very well. That's not where the possibilities end and if you can think of a better idea then you should go for it!
There is no better feeling than a popular Minecraft player on your server. When popular Minecraft players come onto your Minecraft server, they bring their friends. So not only do you have a popular Minecraft player, but you also get anyone associated with that popular Minecraft player.
This also goes for Minecraft groups of friends. When a couple of school buddies invite their school buddies and you suddenly have a daily player count in double digits you feel a sense of pride. Try to promote your Minecraft server to groups of friends make sure they don't quit. When one friend quits he may take others with him! This is also called 'word of mouth' advertising.