1) Look for a monster that I like and refluff it,
or
2) Just give it powers that I think feel Lich-y and appropriate.
For instance, if I
really wanted to get the PCs attention, I'd take one of the powers from one of the Devourer monsters (forgive me, I don't have my books in front of me). The one that lets the monster basically take a PC off the battlefield and suck life from it to power various attacks. I'd give the Lich
that, and target any PC that can throw Radiant damage around or, barring that, target the leader.
That power feels lich-y. It's like the Demi-Lich's gem powers. Either the PC is caught in the Lich's ribcage, or trapped in a mirror housed within the lich's ribcage.
Or perhaps a "Possession" power. Something that allows the Lich to possess a target.
Basically, make it something that not only feels like an undead spellcaster, but one that is the Top of its game. This guy should be unique, in that Could do anything.
3) Spawning minions. Liches should be scary in their own right, but liches have resources. They're intelligent casters who have centuries to craft atrocities and prepare for thieves and intruders. It stands to reason that they'll have some allies handy. I'm planning to add a triggered action to my lich; when he's bloodied, he summons a bunch of undead minions as a free action.
It reinforces the "master of undead" and "always has a back-up plan" qualities that an ultra-paranoid, ultra-powerful lich should have.
The above suggestion is good, but I have another idea.
Instead of Minions, use arcane traps. They aren't minions per se, but the lich's lair is a dangerous field to maneuver, with something in the corner generating bolts of lightning, on one side is sucking PCs towards it and not letting them go, another is a living wall that gets in the PCs way, etc. In order to get to the Lich, one needs to disable the magical traps protecting him.
Or the Lich is protected by a defensive barrier, one that eats various energy types and prevents engaging in melee; in order to break the sphere, they must destroy various magical generators around the room.
Ultimately, the fight with the Lich should be an event; the Lich should be the centerpiece, but remember, he has had an unlimited time with lots of power to design his lair to his suitable needs. He should be Prepared. This is not just some jerk wizard in a robe, this is a
Lich.
if this were a movie or a book and you could do anything, what would the encounter
look like; then design it to look like that.