Using a bit of gas will usually straighten you out...
This of course depends on you having some place to go, without slowing down.
I've had to use it once, towing a friends race car on a trailer. Coming down a long, steep, but straight hill. Let the speed build up a little more than I should, and lightly touched the brakes. Trailer gave the first signs of starting to fishtail, so I got back on the gas and it straightened up instantly.
Luckily there was a big sweeping corner at the bottom of the hill that we cruised around at about 90mph, then just coasted the speed off on the flat.
If we had HAD to stop, things would have gotten VERY exciting.