Rel pretty much gave the same advice I would have.
To expand upon his idea a little bit.
First of all, make sure that at least one PC has strong ties to either family, the local thieves guild or some organization (the more PCs with ties to this group/organization the better).
Have an NPC hire the PCs individually (those with the ties may know one another slightly, but only in the "saying hi as we pass in the hall" way) and bring them together to perform a heist for them. Have the heist be something along the lines of retrieving an item from the local town gaurd barracks.
One of his guidelines is that they may not kill any of the guards, except in extreme self defense. He will give them this guideline saying something along the lines of killing one guard will bring them all down on their heads (i.e. you kill one, you end up with a lot of ticked off cops). The NPC has other reasons of course - he wants as many guards as possible occupied for as long as possible.
Shortly before they perform their hiest, the NPC allows another stooge to tip off the guard that there will be a heist of something that they are guarding (evidence from another crime maybe). This will bring more guards away from regular rounds and such and guarding things that might be stolen from them, making it tougher for the PCs and more likely that they will get caught.
While the PCs are playing with the guard, the NPC hiests something even more valuable from another part of town. This item is stolen from the group/organization that one or more of the PCs have ties to.
If the PCs succeed in their mission and get away, the NPC fails to meet them like he promised (he told them that the item they were after was a powerful magic item, but he lied to them), and they cannot find him.
The following day, the real theft is discovered, and the group/organization start investigating, and eventually discovers that the PCs were used as stooges so the NPC could do the real theft, and then skip town.
The organization gives the PC the opportunity to find the real thief (the "or else" is heavily implied!!!) and retrieve the relic he stole.
Thus they now have both personal and other reasons to be traveling together as they search for the NPC.
Give the NPC some distinctive feature (such as having only one eye or an impressive and very distinctive scar) that will make him stand out. Then you can actually run a series of adventures akin to the old tv show, The Fugative, except with the evil little twists you have included....
Some additional adivice:
1) at first the NPC do not know that they are tracking him, but he will soon discover it.
2) have him hire other groups to take them out as he travels from town to town (alignment is not important as this guy is an extreme con artist and able to talk the unsuspecting into almost anything).
3) eventually, as the PCs defeat those sent against them, the NPC then hires a second group whose purpose is to watch the PCs and to learn all about them and their tactics (so he can better counter them). These watchers specifically watch while other hired groups attack the party (the watchers will not interfere in any battle).
4) as he hires additional groups he will inform them about the PCs and have them adjust their own tactics to better work against he PCs.
This gives you a long term foe for the PCs, and gives them a purpose to adventure (as they try to hunt down the one-eyed Monk with the scar going from forehead to mid-cheek, right across the missing eye).
make sure that the NPC always has some sort of escape plan ready
do not space the NPC related events too closely. give them side adventures as well. False leads can take them to local ruins or other adventure sites in search of him. This will allow you to insert all sort of other adventures as well.
make sure that the NPC gains levels as the party does (roughly) as he is doing other things as well.
use rumors to send the PCs in the wrong direction, or the NPC can actually warn the gaurd that a bunch of thieves have been trailling him and they get arrested or detained as time goes on.
make sure to consider the consequences - this means that the actions of the PCs and the NPC will have an effect on the reactions of others.
One of my favorite tricks is to get the players to give me a list of goals for their PCs. I can then work in clues to accomplishing those goals into the story line. The most fun, as a GM, is when you force the PCs to choose between choose between accomplishing the goal of PC 1 or the goal of PC 2 (note the opportunity to accomplish the goal not selected will be limited in duration, so by the time they complete the goal of the one whose goal is selected, the chance of accomplishing the other is gone...
well, that is all I can think of at the moment....