How to prevent dumplings from sticking together
You can't let the dumplings touch after you make them, just keep them spaced apart until you cook them. I'm not a fan of boiling, I prefer steaming them or frying/steaming as described by @user245427 instead, however if you do boil them you can keep them from sticking using this method:


Use a big pan with plenty of water, that way the dumplings have plenty of space
Drop the dumplings in 2 at a time on opposite sides of the pot so they don't hit and stick together right away
Stir them around often, I use a wire strainer I got from an Asian supermarket, it's perfect for the job and you can lift them out very easily with the same tool. See the pic below




I wouldn't bother oiling them before boiling them because the oil will wash off in a couple of seconds and you're just wasting a good ingredient without getting any benefit. Also, don't flour them, you won't solve the sticking problem without using a lot, and the flour will just be a gooey mess later that doesn't taste very nice.