When that happens, I just take a break from it. Do all your normal things outside RPG Maker XP, and bring a pad of paper and pencil with you around, or whatever.
Then your mind will just jog an idea and you can write it down. Or better yet, look through myths and legends. That always helps me, and people like games based off of mythology, but with your own twisted idea of it.
I'm pretty good when it comes to storyline and characters. I can't tell you how many stories I've been writing ^.^. My friends are always asking for more. Maybe I should have a thread where I can help people to develop storylines. I put a thread for help on one of my earlier games, and found that I was better off just figuring it out myself.