Keeping players isn't as easy as it seems. Generally with the average server, someone who plays on there may only be on for a single day - if not just coming on to see if they like the server then leave.
Wtb Work
The first easy thing to keep players, is to give them work. Blizzlike servers can do this pretty easily with the 1-70 grind but funservers can be a bit harder. If you make a mall with eveything they could ever want, what will they do after?
You shouldn't expect people to buy from the mall then go to straight PVPing. As great as PVP is you can only make a good PVP area once there are players constantly on.
A server I saw had a level grind for 70-99 with blizzlike exp. What they did was make quests for each leveling zone. "Go kill 15 of these, and 8 of those. I'll give you [Sweet PVP upgrade] and [Zomg 5levels]". The thing to note, was that two good 70s could kill a 99. Leveling was just a fun way to make your char stronger for pvp (And a great way to keep players from leaving if nobody is at the arena at that time).
Whoa, cool
Players come to servers that are customized. It is a simple and true fact. And generally (With the exception of chrispee's and knaur's) you need to change alot of things around to get a good server.
Tips: A mall in shattrath? Too many people go there anyways.
Secret troll village? Its really not that secret...
Why not make your own area? Using game objects you can build and entire zone with everything right down to flowers on the ground. New stuff is cool stuff. No other server has your exact custom zone.
Don't stop there! You could make a mall under the sea as long as you have some sort of script to give everyone water breathing! Spawn a big area in the sky with vendors! Or make vendors a challenge to get to! Maybe to upgrade your tier you need to pass a obstacle course, or possibly customize your own raid with vendors hidden behind bosses!
Events
Events are a great way to have return visitors to your server. Keep a calender on your site with dates for events. How about making a GM morph into a creature that must be killed? -Important- Do not make the event "First to get Killing Blow wins!". I suggest giving a event coin to everyone who was there instead (Especially to be nice to the healers).
Proggression
Players want to feel like everything they do makes a difference. If they spend a hour trying to beat a obstacle course, give them something! A tabard with stats or some really good bracers?
If you have a custom instance, how about making coins drop off every mob which can be traded in for custom gear? Remember: Progression means getting stronger, cooler, and above the rest. Make custom gear look good.
Ooo Shiny
You are a admin. You can do anything as long as you put time into it. I suggest using game objects alot. you can even get shiny lights over all your vendors, or portals, or any points of intrest.
http://www.mmowned.com/forums/progra...t-creator.html
http://www.mmowned.com/forums/emulat...ects-game.html
http://www.mmowned.com/forums/emulat...ore-guide.html
Now some of this may seam like common knowledge, but for the amount of servers I see with instant 70 and nothing else... Y'know.