Remove the stinger as soon as you can - usually this can be done easily with tweezers, but don't squeeze the venom sac at the top since this will only push more venom in. When you've done that, wash the wound as soon as you can.

After that, there are several things that supposedly reduce the pain - if you have any baking soda with you (sounds silly but if you frequently get stung it may be a good idea for this reason) then mixing some in with water and applying it can reduce the sting. Note that a common misconception is that all wasp stings are alkali, and therefore vinegar should be applied - *a yellow jacket's sting isn't however, it's acidic!*

Antihistamines can also help with the swelling, so (barring other health complications) taking some of them should help things.

If you have nothing along those lines with you then I've seen several sources mention that a mud pack, mud mixed in with water and then applied to the wound can work well. Presumably this works best in areas where the soil is alkaline.