To supplement old man Broken's ramblings, I would like to say that even if overplanning is perhaps not too great, having a clear end in mind IS very useful, because then, that means you (and the players) have a general idea of where to go in the end. The only RPs I've completed as a GM were relatively short RPs which had a pretty big focus on a quest-like plot, whether it be plunging into the eldritch depths of the Inverted Spire or destroying the goblin nest of Rugome Fort. Both of these had a clear end (everyone died before they reached the end of the Spire, and almost everyone died against the goblins because the goblin leader contracted with an alien god), and also lead to the teasing of future plot threads to pursue and keep in mind if you wish to continue it (what the hell was that last thing they saw in the Spire before their death? why the hell did an alien god grant their boons onto a single frickin' goblin?).

A pattern of having a problem, solving/failing the problem, and then the reflections afterwards forms the backbones of the RPs I've enjoyed along the years. There's always that adage about how the journey's more important than the destination, but having a destination informs the journey and gives meaning to it. Also it cuts down on the dread of an ending RP if you've always been prepared to end your year-old baby from the beginning.

Just don't be so excited that you start talking about how cool future arcs would be, while neglecting to make the starting arc fun and engaging. One, it's spoilers, two, it kills the surprise. That being said, I guess this is more RP-running advice rather than RP-interest-gathering advice. Or perhaps they're one and the same?