If you want to throw a deep post, particularly when there is an inside route designed to draw the safety away from the outside receiver, I suggest using a technique called the “6-3 post.”
The numbers refer to the number of steps the receiver takes: six vertically, attacking the defender’s outside hip, then three quick ones at 45 degrees to the sideline (sometimes with a head turn but not necessarily), with the break to the post made at full speed on the ninth step, or the third of the “6-3.” The idea is that you will take away the safeties either through play action or some kind of inside route — like in the Mills or double-post concepts — while the 6-3 technique will enable the outside receiver to get plenty of leverage as he bursts inside.
See the video linked here for a great example of Kez McCorvey from FSU (remember him?) running the route near-perfectly. (For some reason embedding is disabled, so you’ll just have to click the link.)