Well a few tips and pointers... (Although we already have a bet on this, I'll give you a Ј150... if you get a server up with a unique script. Or you give me a weeks pocket money).
DON'T Refund players! Why?- It's ruins the economy: You get players who have been refunded with everything, and players who join just a month later struggling to make money. Then the players who have been refunded killing the new players like twats in a DM, or in RP as a new player CANNOT defend themselves!
- Score is made useless: Score in a RP server is suppose to tell others, how much you care about the server. How experienced you are, and many other things. There are many people who play for over 3 days actual time (72 Hours) sometimes longer just to get that blast when they get their new score! Refunding this means nothing... oh wow my scores now 102. I really worked hard for that.
- It makes policing hard: This makes policing hard, as you have a tiny little patrol car, your never going to catch an infernus if it's well driven! In servers that don't refund owning this as a tool is a privilege and is fun to own as you can do more dangerous things, with a greater chance of cops coming on scene
Alter SF, not every building to too pretty to demolish... right? Not one city really has everything a good RP server needs this means you need to improvise. Especially in SF, so demolish some things and make greater better maps.
Make Easter Eggs, one of the reason Grand Theft Auto is a bug hit, is because it's full of things to be found. When your players get bored in RP, they have nothing to do. I've always found hiding things around the map... mean they play longer. As they have something to do when no one is online, and they have a blast finding them.
Have hidden features, at the off this doesn't sound great... however I mean like at the top of chiliad. Have a little check point saying "Bring your friends, lets illegally race". This is something to be found, once it's found people can have actual mapped illegal street races, and cops could spike that area looking for speeders. Even if it's not a race, just be creative!
Have world progression, every month or two... change the world slightly. Then when a player reads a newspaper, have this in the news paper. "After the robbery of San Fierro bank last week, the bank has suspended all transactions for the coming months. This has brought outrage in the citizens of San Fierro, some even threatening to start fires in protest until they get their money.". Also actually act this out, and have this date KNOWN so people can join in on this event happening. This used to take my players from 30... to over 120 when I did on one of my RP servers. EVERYONE joined at that time to be apart of that event. Once that finished, the server restarted and the consequences of that act were put in place.
Allow people to do more things, such as actually boost cars and sell them at the crane. This means you can add insurance to your server and your economy. Just don't let the thief excel to much, and make the person who was stolen from lose a little for not locking or securing his vehicle. Just keep in mind SA is set in the 90's I think.
Allow people to make their own stories and hit the news, make the news paper MySQL and have a few moderators working on it. This is a great thing, someone could create a car theft ring... and this could be reported to the news paper on your website. The moderator approves this for the ingame paper, it then appears in game. Then people react to that, like the press is onto us! We have a snitch, that's happened in one of my RP servers in the past and the boss of that ring character killed 5 people. Until that got leaked to the papers and the rest of the gang character killed him.
I could go on but this is getting tedious to write, although the above has served me well in the past. I'm never going to use those unique features again so make the most of it!