Generally I tend to go through a weeding out process with RP's now that helps prevent interest decline.

[b]Step 1:[/b] [b]OMG NEW IDEA OMG AWESOME OMG!![/b] I will wait a week before I write anything down. Seven days.

If I stop thinking about it, it was a momentary flash in the pan, a craving I probably got out of my system with a video game.

[b]Step 2:[/b] [b]Okay! Still thinking about it![/b] Well that's interesting. Write down the premise, a few notes. Simulate what directions it could take with a typical cast of leader, follower, romantic interest, and rival (party of four stereotypes), attack it from several angles. If I find it's very narrow and I can't figure out multiple paths through the story, or keep a grasp of strong characters for the players to interact with, I scrap it.

[b]Step 3:[/b] [b]Premise down! A couple strong NPC's for players to talk to, and they have lots of options available![/b] Alright! Now we're getting somewhere. Time to AFK from this idea for another week.

[b]Step 4:[/b] [b]I came back after a week and noticed critical flaws, but I'm still interested in doing this idea![/b] Repair flaws. Wait two days. Review. Repair. Repeat until it's fully functional.

[b]Step 5:[/b] Finish premise, talk to a couple of my fellow RPer's about it. Describe it to them in as simple a manner as I can, see if they're interested or excited about it, record their questions and see if I can answer them. If I can't, honestly say I hadn't thought about it, work on it, ask them again later if they like it. Continue developing idea with player feedback. [u][b]This step is critical to consistent success, if you ignore it, you're literally flailing in the dark[/b][/u].

[b]Step 6:[/b] [b]Okay! Still interested after three weeks or so, refined the idea, have a couple strong NPC's I like playing, players have multiple paths, I have a core base of players excited about this RP before release...[/b] Now we're really getting somewhere! Build the OOC and whatever supporting information I'll need to start with, release the plot/premise and some description about the world as an Interest Check.

[b]Step 7:[/b] [b]PEOPLE LOVE IT!![/b] Awesome! Lets start this shindig then.

Only about one out of every ten or so ideas I think of survive past step 3, because most ideas I have are flashes in the pan, something neat or exciting in that moment, not something I'll want to keep doing forever. It's kind of like a puppy. If a five year old says they want a puppy, that's very normal. Puppies are cute. Give it a couple weeks without mentioning the puppy and with sufficient distraction, the majority of them will forget all about that cute puppy that they weren't going to take care of, but someone who constantly works at it, who has to wait for it, who adopts responsibilities and consistently plans and talks to people about it, a child who asks what puppies need to be trained, who does chores to buy the leash and collar... -That- kid is going to really love his puppy.

We're just like that kid with ideas as GM's. We want every damn cute pet we see, but only a few of them will really stick out to us over time, and those are the ones you should pursue and acquire when you're ready, when you've thoroughly made sure it's not just a flash in the pan but an idea you're really seriously into.