i just spent the day working on 360s in the park and i've got them down for the most part. when learning i found that flat land 3s didn't help much because you have to spin so fast and try to whip the board around, which is not the same movement you need to do on a 20 ft kicker. to me, the most important thing is the carve up, probably because it is hard for me to translate what i want my body to do and what it actually does while i'm in the air.
i would always open up and land 270 as well until i really focused on bending my knees while in the air and keeping them that way until i was ready to land. i would not straighten my legs until i had done a full 360 rotation. if you throw in a melon or indy grab right when you leave the lip it will force you to stay compressed in the air.
i spent 6 hours in the park today only practicing 360s, and now i can land them 9 times out of 10 when just this morning i had only tried them a handful of times before.
it also helps to get someone to watch you or even video you so you can see what you're doing in the air. it's hard to figure out what you need to improve on when you keep falling and have no idea why because it's pretty much a blur from the lip to the landing.
that's my analysis of it anyways. most important things are a solid carve up and to practice practice practice.