So, when running RPs, I always think it's a good idea to have some storylines saved in your back pocket to help move the story along. Keep in mind the main plot of the RP, and then play off that. If the characters are say... in the jungle, maybe there's an attack from leopards. If they are on a journey to get somewhere, maybe they get abruptly hit by a storm. If they're apart of a herd who desires only peace, maybe an enemy herd attacks them.

Another idea to keep things interesting is establishing a goal for the RP. I've found that the RPs that have specific objectives seems to fair the best because there's actually a point that all the characters are striving towards/against rather then making things up on the fly. One of my most successful RPs, the Alternative Ending RP, had the goal of one of the herds trying to unite all Anok into one big herd by any means necessary. This led to there always something going on, rather that been the conquest of a new herd, or conflict that is bound to arise when pegasi from conquered herds are suddenly forces to live together. Drama sounding relationships also is a great tool to use to keep people interested.