Everything from the way you grip the disc, to your balance on your feet, is unique to you. So without seeing you throw, it's gonna be hard to tailor advice specific to your body mechanics...
Disclaimer aside, I believe there's three things to balance when you're making a throw: power, spin and release angle. Power gets the disc down the field; spin keeps the disc airborne and level; angle allows you to tune the way the disc curves and drops. To get those beautiful, long, floaty hucks that settle down gently, I find you need to be attentive of your spin and your angle (the power is something you can increase or decrease depending on distance).
Think about the way an airplane lands as an analogy. As the plane approaches the runway, the wings stay perfectly level and the plane tilts back, allowing the underside of the wings to cushion its descent. This slows its forward movement and also its rate of vertical descent.
In the same way, the disc needs to stay flat, maybe slightly 'leaning back', so that the air gives it a nice cushion it as it falls. With your release tilted slightly up and by putting plenty of spin on the disc, it will settle down nicely at the end of its trajectory. If your throw is perfectly level, it will probably stay that way and maintain its speed (and sometimes you want that)
And be sure to keep the disc 'horizontally' level to prevent the disc from turning. Even a little turn to one side allows the disc to cut through the air laterally as it drops, so it maintains a lot of speed coming down. Think about how quickly a blade drops!
