several times when running a game i've found the pc's getting into some very large combats of potentially hundreds/thousands of people.  in other words, they are in the midst of a large battlefield and just curious how people here have handled it, both for the players keeping them feeling like they are in the thick of things and on the gm side of it as it becomes quite impossible to really track everyone and everything (hit points/actions etc)
i've done it a variety of ways before, but its always felt off and not really the effect of being in the middle of something huge.
any thoughts?

There's a Finnish rpg that basically blows this sort of thing out of the water, Praedor. It's based on having the GM grind through the battle in general terms and then zooming in on individual scenes with the player characters participating in it, who then get to do stuff like surviving, assaulting enemy leaders, sabotaging something or whatever else one might imagine doing in combat. Success or failure in these small scenes then factors back into the high-scale combat grind. Just allow the players to choose how insanely dangerous they want their characters to be getting, factor that into some random rolls over what sort of opportunities the characters find on the field, and you're pretty much done. Works just fine for traditional fantasy adventure gaming, anyway.

Assuming you're playing a small band of heroes game like D&D, here's the way I've done it.

First, you take out markers to represent different squads of guys.  Then you make a map (for me they were always very crude) of the battlefield and place where the enemy soldiers were.  I then told them what forces they had and let them position their forces and give them a direction to head.  I then let them put themselves somewhere and decide where they were going to fight.  I then referred to my secret "enemy commander strategy" and figured out which groups clashed where and how they'd fair.  Then, I'd have the players play a battle with whatever enemy squads they'd have encountered.  Keep in mind that "partial victory" is very common on the battlefield.  Look at Napoleon, for example.  He won every battle he got into, but so many troops died that he lost the war in his final battle.

Several RPGs (Flashing Blades, Bushido, 7Th Sea, Legend of the Five Rings, Roddle of Steel) do the large scale conflict before or parallel to the players specific scenes, which can in turn affect the larger combat sometimes.

I have always had a terrible attraction to running wars with some sort of war game, and depending on the feel for the players I want to evoke, either using it to suggest moments in their experience of a battle (from boredom, to totally screwed), or to allow for opportunities to either seize glory or even affect the battle.

Once while trying to explain abstracted conflicts to my wife, I suggested that whatever factors you think matter in a skirmish, battle, or war could be rated on a simple scale of how important you think it is. Everything from technology, civilian and military morale, technology base, excess and regular population size, military culture, sense of purpose could all be rated on some large scale, like 1-100 each. Total all values, divide by 10, roll that many d10s. Inflict that much to the rating of the other side as THEY decide. If you want to target something in particular the other side gets to do the same.

You can get way mnore complicated, with certain technology values multiplying the value of soldiers or whatever, and you can require tactical checks and warfare checks to see who gets how many concessions etc, but for story telling it can work out just fine.

Look at Napoleon, for example.  He won every battle he got into