how to keep players on a darkrp server?

you should be good at glua and use good addons or you can write your own.

motivate them by making events and novelties

Try to do events, giveaways, have some entertaining addons, have a good map and and have a good staff team that people will want to come back to.
Keep it clean and engage with other people. That all i can thing of.

Make custom content and keep updating every now and then, run events and keep the game interesting

I've ran many communities in the past for a plethora a different game modes. There are a few things to note.

Staff
What does your staff team look like? Untrained staff can make a large impact on the outcome of the server, making sure your staff team is trained to deal with certain situations and that they are unbiased in all sits, using factual evidence such as logs. I'd recommend if you haven't already, setup some basic guidelines for your staff to follow. One thing that I usually do is allow the community to view and +1 or -1 the app, my past players have seemed to like this "system" and it worked out for me many times before. Keeping the players happy and weeding out bad applicants.

Content
Now this one is touchy. It depends on the game mode mainly. certain game modes are rightfully content heavy such as FalloutRP, HaloRP, HL2RP etc. But the thing I know very well, no player wants to wait 30mins+ in the loading screen while 10gb's of content is shoved down their throat. Either optimize your load time or don't use as much content. No player wants to download content they will never use or even see.

Host
This ones also important. Purchase a host in your region. If you live in California get a NA west coast host. If you purchase a host in the UK and you like in California your ping will be so bad its almost unplayable sometimes. Make sure your servers hardware is up to par with your game mode. If your running a content heavy game mode like the ones stated above, your probably going to want to average a large amount of players to use the massive amount of content. The reason I'm saying this is some hosts offer you specs with unlimited slots, meaning any package you select will allow you to have a 128 slot server, and upgrading would mean increasing the efficiency of the server itself.

Developer
Do you know what your doing? If your server is a buggy mess where nothing works, people aren't going to play. Make sure to tune everything to your liking as a lot of things can be easily exploited. I'm not saying go find a developer, but if you know nothing about cookin' up a server then take some time and learn.

Other then using common sense, I don't have anything specific to note. If your having trouble finding players hit up your steam friends, discord friends, advertise in other discords that allow it and that might bring in some interested players.

Make it enjoyable, unique, and definetley dont cluttered it wit

You must make your server enjoyable without any leaks or anything and make your server very fun to play with all your friends!

make an enjoyable server for players

Interesting addons that fit into your server and the dark rp mod itself, adequate administration only through calls and requests

DarkRP - a game mode for the popular game Garry's Mod with the idea of a simulation of urban life. You could become a gangster and rob