It is about just messing around with small parts in interesting ways. Also, having a lot of one piece is always helpful for interesting greebles. If you find an interesting piece at the Pick a Brick wall, be sure to grab a lot of them. Once you have them, try out all the different ways they can attach to other pieces and each other. If you haven't seen his stuff already, Peter Reid is a fantastic example. Good luck and happy greebling!