One way is to use either folding or solid molded paddle clips As shown below.
Probably the most pupular type is one made out of bungee cord, 2 deck loops and a hook.
This is what i use and once you are used to it you dont even need to look to be able to get the paddle secured in it. You string the bungee with a loop on ech end into two deck loops and stretch it slightly. Then put the hoook somewhere that it will tension the cord when pulled over the paddle shaft to hold it secure, but not to tight as to make it hard to pull on.