What I usually find that helps is watching and studying the plot of some good movies. Yeah, you need a plot (then there wouldn't be an RP) and a seemingly unreachable goal which is then achieved by smaller but significant goals. The kind of RPing I do COMPLETELY steers away from magic mainly because you always have at least one person who thinks they're a real cowboy and can do whatever they want. For example: "Oh, we need to get all the way across the country to pick up this really special order of chicken fries." Then this guy comes in: "Yeah we do. We COULD hitchhike or walk, run, but why don't I just teleport there!?" *ZING* "K, I got it." Yes, RPs like this REALLY upset me. You have the oppertunity to go on some whole misguided adventure somewhere and have all this cool stuff arise, but no, there's always the guy who can teleport or just make it appear AND RUIN IT. Where's the fun? Making things harder and more challenging make things interesting. Aimless fights with magic and powers make me facepalm. They're something about them that seem really pointless and stupid to me. If I wanted to watch a round of Yu-Gi-Oh, I'd go do that. I don't need an RP to do it for me. THAT is what makes me want to leave.
Interesting plot lines could include going somewhere and having your best friend fall in love with your sister and you may not like that but it turns out, he was trying to ruin you life the whole time. You find out WHY! and he could end up killing your parents or something I don't know. Yes this is random but I'm just taking shots in the dark. Ever seen the Godfather or Goodfellas? Mafia furs? You got the rookie cop, crime boss, goons, arson, murder, action, mystery! There is so much you can do to keep things going.
But I must say a lot of RPs die because of people failing to post on time. You could have the most awesome plot going and it stops at the climax because someone decided to go offline for a month. When I controlled RPs (when was REALLY into it) I never waited for people who didn't care to show. You don't show for exactly 3 days w/o notice, you're kicked from the story. That's how I ran things. In the OOC, WANTED: someone to fill in for ______ If no one filled in, I played the part myself. I had an RP go for 3-4 years called Pacco Di Lycan (my record on HumanPets) with about 74 people playing different characters and it was based off the epic movie, Death Sentence. It is possible to achieve.