This confused the hell out of me at first, but seeing it like this makes it easy to remember.
Arrange the objects in a circle, in the order of the title, Rock, Paper, Scissors, Lizard, Spock.
Each item beats the next one item on the circular list. Is defeated by the one after that, defeats the next and defeated by the last.
I have no idea what the hand movements are for lizard and Spock!