'Ammo shell' style dilator kits in graduated sizes help greatly. As do inflatable knotted dildos, ie. something that goes in small(ish) and pumps bigger after its inside you, mirroring actual canine anatomy. Of course it can also be said that simply going up a size or two from the actual diameter of the dick you plan to take, is a good bet too -- the wider you are, the less it'll hurt you. I always liked to think of that pleasure/pain dynamic in an SNR (Signal to Noise Ratio) analogy: you can't hear the music (pleasure) thru all that damn static (pain) so squelch the latter, maximize the former.