An adamantine brick. To be burned or painted black and then hurled into the depths of the Negative Energy Plane, where no one will be able to see the black object amidst the black void.
Alternative: As above, but placed deep within a chamber inside a juggernaught (MM2) or huge stone golem (if one could be fashioned; it's not in the rules), to be burned or painted black and hurled into the depths of the Negative Energy Plane. Commanded only to fight back if attacked. The coffin-like chamber, in which my phylactery would rest, would hold an adamantine box (for reinforcement, in case the golem/juggernaught is destroyed) and also contain several advanced Xeg-Yi energons (MotP) stuck therein by Planar Binding spells or the like, forced to guard my phylactery against anyone who enters the adamantine box except myself.
Other Alternative: An adamantine brick, placed within an adamantine coffin, fully sealed and several inches thick, to be sunk into the deepest, darkest part of the River Styx. The adamantine coffin is to be filled with the most potent and deadly poison gas found in the multiverse. The inside is to be lined with the remains of destroyed mummies, including much mummy dust, with which to (hopefully) infect any intruders with mummy rot. Preferably a small ice paraelemental or somesuch will also be Planar Bound inside the coffin to fight in defense of my phylactery should someone, somehow, find it and make it inside.