Tim's answer is great, but I want to expand on it. This especially applies if you're thinking "NPC villain" and not just "powerful monster."
Liches are people, too
Start by remembering that a lich was a person... human or not, it was an intelligent creature that, for some reason, went to great lengths to cheat death. When you think of your lich as "person with some serious obsession and lots of power," he can become a much more interesting villain.
This ties into Tim's "obsession" issue... why did he cheat death? What was so important to him that he expended so much of his time and resources to become an undead monster? Maybe just fear of death... some reason he doesn't want to cross over to the other side. Considering that in typical fantasy RPGs, "the other side" is mostly a known quantity, what horrible fate did this guy expect to receive? What did he do that guaranteed such a miserable afterlife that he'd rather continue as an undead than move on? If that's all he's really afraid of... what does he do now that he's secured his safety from his biggest fear? Imagine a lich, bored to tears (if he had any), with nothing to obsess over but the safety of his phylactery. This might be the kind of lich that just "sleeps" for centuries at a time.
But maybe death was just an obstacle to getting something done... again, Tim's "obsession." But obsession doesn't have to be a journey ("learn moar magics!")... this lich's obsession could have a destination in mind. What thing did he need to accomplish so badly that he was willing to cheat death and become a monster to achieve his goal? Imagine the lich who is really just a loving husband, who has spent 1000 years trying to find a way to bring his lost wife back from a fate worse than death. Imagine the reunion when he finally succeeds and she sees what he has become.
It doesn't have to be so romantic, of course... maybe it's just power. What's the point of being king if dying means you lose all your toys? The lich king is a common theme... instead of holed up in a tower, this lich sits the throne and runs his kingdom with a dry, cold fist of iron. Which means, in additional to spending those sleepless nights researching more magic, he's got a kingdom to run and his day is much like that of any other tyrant. And even though he knows he can't be killed, imagine his worry about what happens to his kingdom if he disappears for a week while his body regenerates?
Regardless of why he did it, this former person, had a reason to cheat death... and once that pesky death issue is out of they way, he's going to keep right on pursuing his original goal. He's just as intelligent as he was before, just as resourceful. Imagine that he'd simply gained immortality instead of becoming undead... how would his plans proceed? He's not going to enjoy a normal life with the wife and kids (being undead is hard on relationships, but then so is his level of obsession), but isn't the lich going to be doing pretty much the same thing as if he were simply immortal?
Aside from the guy whose end goal is cheating death, think of lichdom as just one milestone in this guy's overall plan. Put yourself in his shoes and ask, "What do I do now to get closer to my goal?" And if he's ancient and still hasn't reached his goal, he's probably bat-guano insane, so there's that to play with.