Generally, I'd say lock it in a box which is a trap that casts Nystul's Magic Aura, Misdirection, and all the other anti-divination spells you can think of on both the box and the phylactery. Then, paint the box white and drop it on the Positive Energy Plane, where visibility is 5 feet, IIRC. A lich is probably capable of surviving the damage for the round or two during which they'll exist on the plane after respawning, so I don't really see any downsides. Also, it really helps to have non-technical defenses. Construct an elaborate, trap-filled tomb-fortress, ensure that rumors about it and the fantastic wealth contained therein, etc. Make it so most people don't actually BOTHER trying to figure out where the phylactery is, because they assume they ALREADY know where it is and it just doesn't occur to them to look elsewhere. Oh, and make sure your head minion carries an amulet that you act overprotective and mysterious about.