If your only spinning 270 its because your not looking around. 360 is the easiest trick in the book. Just pop and look around until ur at 360. If you come short its because your not looking around the last 90.
I found that i could never get around and be level to the ground by looking over my shoulder. Id always go like you and then id be leaning back to far and almost get around but then id always fuck up. Then i tried looking under my shoulder and that kept me perfectly oriented to the ground and i can still see where im going. Now i can do them by looking over my shoulder but i dont let myself lean back, but you might want to try the under your shoulder method, it got me rollin on the whole spinning thing...
And on the 7th day, God stepped back and said "There is my creation, perfect in every way....Oh dammit I left weed all over the place, now they will think I want them to smoke it. Now I'll have to create republicans."
The biggest thin for me to get the whole way around without being to backseat or anyhting was to try grabbing. Try for a safety, and even if you don't get it, compacting your body seemed to work for me.
Having a wide stance just before you come off the lip helps me spin a lot. As you pop and come off the lip bring them together and for me it helps me spin a lot faster. Bigger jumps help too. Once you get them on small jumps step up to big ones, and then you'll realize its way easier off big stuff.
Come into jump with legs about shoulder width apart. Throw the spin with your uppper body (arms, chest, head) as you pop off the lip. (Be sure and pop, don't suck it up. Also, don't take off early.) For a big spin, I'll use the inside edge of my right ski (when spinning left) to set my rotation. From there, just keep your head in the spin till you can spot your landing. If you pull things in tight, you'll spin faster. However, be sure and look at the horizon and not down so you don't go off-axis.