i've found that the best way to think of ideas for screenplays is to just not try. usually a great idea will come when you least expect it and from somewhere you never thought of.
or, if you're not the waiting type, you can write down a list of some cool things and try to develop the best ones into screenplays.
if you still have trouble, you can team up with someone you know to try to think of something. two heads are better than one, after all.
