I have also been using magnetic mounts and have tried a few. Most hold pretty securely with a thin case like the Apple Leather. I just place the plate inside between phone and case without using the adhesive as others have said. The only problem I've found is with thicker cases, specifically for me the Otterbox Symmetry Leather Edition. A lot of magnets are not strong enough to grab the metal plate through a thicker case back however I have found two that work: Scosche MagicMount XL and MagicMount Pro. They are both advertised as having stronger magnets and they really work. In fact of all the magnetic mounts I've tried the Scosche line has been the best and they are not even that expensive. Even the standard MagicMount is great and stronger than most cheapies.
Using the Apple Leather case I've never had the phone fall off a magnetic mount, they are all pretty strong.