1)Not many people like servers with a massive maxplayer limit like 30 or 45, most people will join a server under 20 slots. People think that a >20 server is messy and is too crowded to RP or Build on properly. Get a 16/18/20 slot server instead and you will see more people will join.
- Also ONLY have the most common addons (Wiremod, PHX and Wire Extras. It doesnt matter if you think they are cool, other people may think they are a waste of time), no-one likes stitting around for ages downloading content from a server only to find out its all useless crap that they will never use elsewhere.
3)Run a popular map; like gm_construct, gm_flatgrass or rp_downtown_v2.
If you are running DarkRP add some custom classes and shipments that will be unique to your server (Everyone likes a custom class).
Add tags to show what is on your server e.g: wiresvn, phx, custom classes, custom shipments, darkrp. This will broaden the chance for people to find you and want to join you.
Use a good server name that will catch peoples attention and want to use your server over the others.
Reply if this guide has made more people join your server than before.