be more spontanious! That's why I've been taught during my drama classes and when I attended some stand-up comedy sessions. There is a good book called 'Improv' which teaches you great methods and games you can play with friends to help make you more spontanious and improve your imagination (as that's what improvisation is all about).
As kafine stated, drawing really helps and so does writing. A good exercise is to pick an object and then write about it non-stop for about 2 mintues. Don't hesitate or think too hard. This will teach you to be spontanious and let your mind be more open.
Good luck.