Ancient Chinese Secret.
Lee case lube. Apply it and let it dry, then size them.
Or, what I do is mix 1 tube of Lee Sizing wax/lube with 500ml water then dip all my cases in it (either individually or all at once) and then lay them out to dry (overnight-24hrs?). Then I size them. Works good, no dented shoulders, and it puts a thin film of lube on the entire case, even inside the mouth. It's only wax so it doesn't/won't contaminate your primers/powder.
I tumble my cases after sizing/trimming to make them shiny again but if you didn't, by the time you got around to the point of actually shooting them, the dry wax will have flaked or rubbed off anyways and they'll just have a dull finish.
I'm not saying that there aren't better lubes or waxes out there, but this is a fast, cheap, and easy way of lubing your brass that has more than satisfactory results.