If you use a key pad, make sure it's close enough to close the doors when they're open away from the pad, I've found the best way is to put the enclosure facing sideways to the rift, if you get to far from it it heads back towards the rift and then it will just hit a wall instead of flying back out the open doors, like this, 90% of the time you don't need to close them. If you're soloing getting milk, clear the area around the pin then get the lowest level female you can, preferably not lightning.
I got a level 150 lightning egg and I was so excited to hatch the egg...that when I went to put the egg down I accidentally ate it instead. :( ps4 users: R2 ISN'T GONNA DROP THE EGG YOU'RE GONNA EAT IT LIKE ANY OTHER EGG. REMEMBER THIS. LOGIC BEFORE EXCITMENT.