Throw no more than 45 degrees from vertical. Proper layover will create a lower, more controlled flight. Not enough layover, see Effects of Layover. Each model requires a different degree of layover. A good rule of thumb is as the wind increases come up closer to vertical.
Distant treetops are usually a good rule of thumb.
Too low of a release angle creates a high-climbing end of the flight. A touch more release height will create a level, more consistent flight. Determine wind direction and velocity. Depending on the model throw 40-60 degrees off the wind. If the boomerang lands in front of you, throw more into the wind.