I've been growing them for about 4 years, so I'm certainly no expert, but have read many of the posts here and one thing I do know is you can't kill'em by over fertilizing.
One of the most mentioned seems to be Ozmacote. I use a regular 10-10-10 spread around the base. I have 2 cavendish which I overwintered and had bad frost damage. I spread maybe 4 cups each around them and they seem to be loving it. Both are about 8 feet and growing full steam.
Once the bloom starts I'll switch to potash (0-0-60) which I was finally able to find nearby.
Hope that helps.