Two keys that I focus on for the drop (in addition to all the previous admonitions about keeping the upper arm connected):
--"Hands above bells." Don't flip the bells out of the rack or pull the hands under the bells. "Unwind" the wrists/forearms, keeping the bells below the hands.
--"Make space" and "counterbalance." Keeping the bell close to the body is good but, IMO, not the whole story. Just dropping the bell(s) straight down as close to the body as possible makes it very difficult to get a good back swing. You can get away with it okay with lighter bells, especially singles, but with heavier bells and doubles it leads to an overly abrupt catch at the bottom, putting an excessive strain on the back, shoulder and grip, and making it hard to change the trajectory of the bell from straight down to back into the hinge. The bell needs SOME space away from the body to create an arc and allow for a nice back swing. So you have to find a balance between enough space and too much. One way to do this is to keep the upper arm in tight so the radius of the arc is from the elbow to the bell instead of from the shoulder to the bell as in a swing.
With heavier bells, and especially heavier double, this still leaves the bell too far in front. If your body is vertical, all the space between you and the bell is in front of the plane of your body. So an additional technique is to counterbalance the drop by leaning back against the force of the bell. This allows you to keep the drop closer the the base of support, but still create space between the bell and the body. Another mental cue I use for counterbalancing is "get the slack out." The idea of this is too create some tension between you and the bell (like leaning back against a tug o'war rope) before the bottom of the drop. This gives you better leverage to guide the bells into the back swing and helps to absorb the force of the drop and transfer it to the hips. You don't want the bells just dropping vertically with loose, noodly arms and then have to abruptly absorb the full force of the drop at the bottom.