those are nasty little bastards! i'd like to know this also as i have a few nests in my yard as well.

When you say "yellow jackets" do you mean bumble bees?  Yellow jackets are small, yellow with black bands and NOT hairy.  They usually do not nest in grass, but in/under something. They are the ones often seen at picnics and in garbage cans.  Bumble bees are large, same colors, [u]hairy[/u] and nest in long grass, on the ground.  Neither are territorial and can have several different nests in the same area.  

People's bodies react differently to being stung.  I have been lucky and have less and less reaction to each sting, with no swelling at all.  A mosquito bite is much worse for me.  A bee sting hurts (bad!) for a few minutes, then just goes away, but itches the next day as the damaged tissue heals.  My wife is going the other way.  She swells more each time and has to keep an epi-pen handy.  

Usually, when mowing over them they get you on the next pass, not when you go over them.  A tractor is never so slow as when mowing over the little bastards!!  LOL  One word of advice--if you have mowed over them, and see them ahead, just leave for a half hour.  Barn swallows, and other birds will have them all eaten in about a half hour.  Your only recourse when attacked is to run.  Get inside and smash those that are on you.  It is very terrifying.  [shock]

Okay, good, now that we have a positive i.d., it is easy to deal with them.  They can and do live underground.  When you locate them, mark the spot, using whatever means is available.  Throw something if necessary.  Get some liquid insecticide (malathion, diazinon, dursban (if you can find it--it's best, but outlawed), and mix up a gallon or so, per the directions.  Put it in something like dumps quickly, but accurately, like a watering can.  Go out [i]in the dark[/i] to the spot and just use a flashlight to find the spot, then turn it off.  Dump the poison down the hole and you win!  Do this at night as they cannot see in the dark, and they are all "home" then.  During the day, roughly half of them are out foraging.  Now that you know about them, always pay attention when in your yard.  When you notice one landing on the ground, go and look from a safe distance--5 feet or so--and mark the spot.  Killing commences after dark.  [}:D]

Here is another pain remedy to be used immediately following a sting. Soak a cotton ball with vanilla extract and apply it to the sting. I thought this was load of crap when I first heard of it but it really does work.

The method beekeepers use to deal with stings (on civilians--we're supposed to be able to take it--LOL) is ammonia.  Immediately rub it briskly into the hole, after removing the stinger if it was a honey bee.  There is a commercial product that is similar to a Magic Marker, but is filled with ammonia.  I think it's called Sting Kill, or Sting Stop.  Ammonia neutralizes the venom and stops the pain almost instantly.  Ain't science wonderful?  [:D]

Another neat way of getting rid of a nest of these buggers (or any wasp-like creature) is gasoline.  Down in South Texas, if I find a wasp next in a shed, or on an overhang, I just throw a half a coffee can full of gasoline on them.  Direct hits kill them instantly.  Just don't light a match!

Although, I'd bet that throwing a bunch of gasoline in an underground yellow jacket nest, and then tossing in a match (or tracer or incendiary round!) would be pretty damn cool [rocket]

-Gloftoe

I heard that hitting your thumb with a waffle cut framing hammer gives the same result[:D]

For me, tobacco and saliva works well. Just moisten the tobacco with some "spit" and apply (remove stinger if necessary) directly to affected area.
I did this just last Friday to two stings, and the pain was gone in a minute with NO swelling.