Alright, hey there!
I own an MC server as well! It had an average of like 4 people online, until I found out a great trick. I kind of quit Minecraft though, and haven't joined my server in two weeks due to school, so the amount of people might have went back down. However, I have gained over $3000 in donations over the summer, and the whole experience was very fun.
Here are some server lists that you want to advertise at:
Put your server on all of these server lists. In order for it to become popular though, you MUST Get the Votifier plugin and the GAListener plugin.
PM me if you want a full tutorial on how to install Votifier, (or add me on Skype: Gman_HAX)
Anyways, 'bump' your server every 24 hours on Planet Minecraft. Make sure the title is very descriptive and interesting, so that you get more people on your server. Also, make sure to make a banner for your server. Choose 468 x 60 when choosing a size.
Lastly, this is the most important step to boost your server's player count. I used this on my server, and it works completely. The simple answer is YouTube! Make an amazing trailer for your server, or even just a walk through of the server, and upload it to youtube. As you may think, the video won't get more than 100 views. However, there's a nifty trick to get you tons more views!
The answer is: http://www.ytnuke.com
I used this site for my youtube video - they always give you MORE views than you paid for! Yes, I know, these views are "fake." But they are distributed across the globe. This means, once your video gets around 10,000 views, it will show up in the search bars and related videos for all of the popular server videos! Yes, I did this and my player count on Saturday reached 48 players; an amount I could only dream of! I also made $600 on that Saturday, in donation money.
Here is the Youtube video for my server: http://www.youtube.com/watch?v=XT0baPw4Udo
If you want, you can also join my server! IP: mc.destructionpvp.net. I'll also tell some of my players to join your server for a fun time :) Also, make sure to keep your server updated. Mine isn't right now, since the new release of bukkit isn't out yet. Anyways, happy crafting, and I wish you large success with your MC server! :)
Professional server owner, and have been playing Minecraft for over 2 years!