I came here for this exact question and as I mess around with the replies I have discovered an improvement on what has been offered. By improvement I mean less keystrokes and the method is very easy to transfer to multiple drops at once.
The trick is that instead of clicking on the silhouette of each unit you can simply click unload all and target the transport itself!!! Additionally the waypointing already mentioned above can be used to queue the entire process up.
Will multiple transports you just need to target all transports with the unload all action.
Here is an example,
If we have 3 overlords full of roaches and want to drop them all in the enemies base, we would like to start dropping as soon as we cross into the enemy base and keep moving to drop the subsequent units further into the base. The keystokes are as follows:
Move, Shift + (d, Lclick@overlord1, Lclick@overlord2, Lclick@overlord3, Rclick, Rclick@ground)
Translated this would tell your overlord to move into the edge of the enemy base, you then hold shift to queue your actions, d selects the "unload all" command and you have to left click all 3 overlords, followed by a right click to cancel the unload all cursor and finally another right click on the ground where you wish your overlords to move to (further in the base).
I am going to go ladder and see how my opponent likes 3 perfectly executed drops going off all over their bases at once =D.