Make a more appealing ad.
Have as little jobs as possible. Just keep the core ones: Mayor, Cop, Gun Dealer. Those are all the jobs you need, let the players decide what other job they want to be using /job.
Enforce roleplay, and have a good administration team. I’d recommend having a welcome menu when people join the server, containing a MOTD and lettings players set their RP name and model before spawning.
Limit guns. I can not stress this enough.
Have just a few very basic guns on the server, and make them expensive and hard to get. This will cause less deaths.
Something else you might want to do is code in a ‘faint’ system similar to the one in L4D. When a player loses all his health, he’ll drop to the ground and start slowly bleeding to death. From there, someone can revive him, but if he isn’t revived in a certain amount of time he’ll bleed to death. If he doesn’t want to wait to bleed to death, he should be able to just type in “/acceptdeath” or something in chat.