Players love competition. If there is a way to be better than others on the server, most people will try for it. Goals to work towards, rewards for completing certain jobs, or even just a simple display of player statistics, are all great ways for players to compete with each other. This also encourages players to play more.
Of course there will be inequalities as your server grows (different amounts of money, different ranks etc), but make sure that players are not getting huge advantages that make gameplay unfair. For example, on a DM server, giving miniguns to high-ranked players will annoy low-ranked players who don't have the chance to move up a rank because they are being overpowered by miniguns, and these players won't want to return to your server. "