I've talked about trying to make your game world seem alive and persistent I don't know how many times. I've talked about ways to do this, and I've given various bits of advice on things that can be done. However, whether you're trying for a living world feel, or just running a game where the PCs tend to hang around - or at least return to - similar areas, you need to keep track of your NPCs. I'll admit, it's not an area I'm super great at, but here are some of the methods I've attempted thus far, with mixed results.
Whether or not you are trying to keep track of NPCs, session notes are a good thing to have. You don't have to cover everything, but jotting down a few notes about what PC has done what, what fight happened where/when and for what reason, and what certain PCs intend to do. When it comes to tracking NPCs though, session notes are just as good a place to start as any. If the PC meets an NPC you think might be important - or that gets a small amount of screen time - then mark it down. If you give the NPC a name, write that down as well. Then, in future sessions, if the name or details are needed you can just go back and check your notes.
The downside to this method however is that as you get further from the session an NPC was introduced it becomes harder and harder to find where the notes of this particular NPC is to be found. Notes you can't find are just as bad as notes you never took...
I've toyed around with this for a while, but essentially it boils down to this: excel is a powerful tool that can track a lot of data. So how do you do this? After you've introduced an NPC you enter them into the spread sheet with all the details you may need for them. You then have a tool that is handy enough to sort and search through and a form you can put almost endless .
On the down side though, this either requires you have a computer at the table (in which case that can be a distraction, but also read the next section downside) or you have to remember the important details until the end of the game when you can finally get to a computer and type in your notes for the NPC.
Note Cards are the method I'm using right now in my current L5R game. When an NPC gets named I make an index card for them. I classify them by their clan/affiliation (clan name, ronin, peasant, etc,) and the fill the card with notes about the person. When I need them to pop back up, I grab the card and I am back where I started.
On the down side this is more work than it seems, especially for longer games. You either have to remember the details until the end of the session, or you slow down the game while you fill out the card. Also, and the thing I've been messing up lately, is you have to keep the cards up to date and use them...otherwise, what's the point?
Do you have a method you use for NPCs? If so, I'd love to hear it.