First of all (this is something a lot of servers fail at):
don't open the server too soon. people tend to ask owners to open asap but let them wait. A bad start is very bad for the longetivity of a sever. Make at least 1 or 2 closed beta test with different people (always the same people tend to overlook bugs). A bugfree server at start is impossible but when there is an exploit people will find it.
Secondly: Advertising, a lot of people tend to do too little advertising. Make discussion thread,advertising thread, mapping thread,screenshot thread. go to different sites. maybe even if you got the cash buy a countdown/banner on otservlist (i wouldn't suggest this for a custom server because they are really expensive and sadly enough these days you won't get enough people to break even)
the next thing is:
Listen to your playerbase BUT think twice before you make game changing changes, people tend to ask for stuff they benefit from and then learn that it ruins the server, i've seen it happen again and again and it makes me sad
Keep the players busy: This doesnt mean you have to double the content every week but make some small changes, some new spawn, some new secret OR just have some cool
event. A lot of server tend to focus on adding new content but you can also just keep players entertained with a decent event: Eg. Easter eggs scattered around the map, try and locate them (if you click on them maybe you'll find some cool unique deco). Or The kings dog ran away, it only listens to the name: Snuffie. Ask around and try to locate him. the first one to use collar and succeeds (10 percent chance) will get a royal gift.
Also what ive seen on some servers and i really like: Make the map bit bigger at the beginning (outline) but leave some stuff blocked by boulders or so. when you are ready for a release say: An earthquake southeast managed to do a lot of damage and a lot of the mountain moved. (make some cool story behind it) then people need to think and explore for themselves.
Last but definetly not least:
BE MOTIVATED! Have fun at creating a server and working for it. it doesnt matter how many people are playing it, if you're having fun and keep working at it , it will succed. Look at Tibianic: it had some down periods also but Iryont kept working on it and look at it now ver 500 people at peeks! as long as you stick to it and keep working people will see that the server is here to stay and will come back or try it again.
Sorry for the wall of text
Good luck with your server!
Kind regards,
Hehemidu
