Need help in writing a story - many stories going on at once?

I'm a screenwriter and up until know I've been writing very simple and linear screenplays, of which I've turned into short student films as well. For my next screenplay, I want to do one in which many stories are going on at once but end up tying together at the end. So far, I'm really stumped as I can't find a way to make the different stories coincide. Any help extremely appreciated.