Best ever is probably to spin the pedals;

But today I got a really awesome mtb tip. It was rainy and the roots were really treacherous, and my back wheel was sliding all over the place. My buddy had me loosen up the tension on my brake cables so that there was more play in the levers before the brakes engaged (in other words, you have to pull the levers further towards you before the pads grab the rim). Consciously or subconciously, you grab the brake levers when you're in a situation when you want a better grip and by doing this you end up grabbing LOTS of brake and the wheels can lock up. After adjusting my cables in this manner, I noticed that I was able to brake more subtly and effectively and when I grabbed the bar for more stability I only got a little brake instead of a whole handful. After that the roots became a lot less of a problem!