There is a pretty good chance that you will have crossed your fingers as a way of believing that it is going to bring you good luck in some way, but are you aware as to the reasons why it is seen as making you luckier?
The origins of this simple gesture are religious in nature, and it is all to do with the cross and asking for divine protection from God. This was originally done in secret, as crossing your fingers to represent the cross had to be done in such a way so as it did not attract the attention of Pagans who would have certainly had something to say about it all in the early days of Christianity.
So, basically the act of crossing your fingers was an attempt to stop bad luck from affecting you more than good luck and it was certainly a protective thing to do. Now, we just relate it to good luck with most people oblivious to its religious beginnings.