I love, love, LOVE hiding phylacteries. I've come up with a bunch of tricks for them, some flavorful, some just mean. The trickiest one I've ever had was an Alhoon (mind flayer lich) who took his phylactery to the center of a featureless desert and cast an Imprisonment spell on it. Scrying attempts were futile, because the party received alternately an endless dessert of a bunch of rock. It took them three castings of Contact Other Plane (at something like 17th caster level), then a month of combing the desert to discover the spot they needed to cast Freedom at. Maybe a little too tedious in retrospect, but a durn good way to hide a phylactery.
My best phylactery ever, though, involved using a variant of the Disciple of Ashardalon prestige class from the Dragonomicon, in which a dragon learns to replace their own heart with a demon to attain immortality. This lich had taught a Styx Dragon how to use the evil of his phylactery to sustain it, so the party had to travel to Pandemonium (from the Greyhawk cosmology), find this dragon, then fight it in the middle of the river Styx. By the way, combat in a plane with objective directional gravity is awesome.
In any case, the important thing to keep in mind when hiding a phylactery is that even though a lich is usually pretty paranoid about it, they're also pretty smart, and arrogant. Despite most books referencing the lich keeping its phylactery near at hand, this is just plain stupid on the lich's part. Unless your lich is characterized by extreme paranoia, most will likely hide their phylacteries, far, far away from where they usually lair, so that if they are slain, they'll have plenty of time to re-form by the time someone tracks down the source of their existence. Of course, magical defenses are a given, and most lich's are probably arrogant enough to believe that whatever defense they've left in place will be more than ample - they won't loose too much sleep (figuratively) over having their phylactery out of arm's reach.
Don't go...
Depends on the rule set your using... My FAVORITE version of the lich is from the 2E Ravenloft Van Richten Guides...
In THAT version, the rules played in a way, your hiding would be terrible...
The lich changed his body, and bonded his soul with the phylactary...
Any time the body is destroyed. the spirit skips the afterlife and goes right to the phylactary 'magic jar' style...
And there it waits for the next body in XX range. It could take days... it could take years... Doesn't matter... the lich has forever!
THIS way... the phylactary has TWO requirements... It has to be hidden and protected... but it can't be TOO impossible to reach. Mixed in with a bunch or treasure is usually pretty good... even if Adventures wander off with it... SOMEDAY there will be a body nearby... and then vengence begins anew!!!