Depends on how you want to approach the situation.
1. The best pointer is to be prepared. Make sure somebody knows, before you go wheeling, where you are and the trails you may be on. Have good functional equipment, full tank of fuel, good tires, water, food, blankets....
2. Usually it's best to try to avoid the storm, or out run it or you can wait it out, if you have the time. Avoid the canyons due to flash floods if you know thunderstorms are in the area.
3. Try to pick the best course, remember momentum is your friend. keep rolling whenever possible.
4. Watch for rocks, fence posts. trees, tree roots. Sometimes they can be hidden. These can grab you, tear your tires and you'll be waiting it out or changing a tire in nasty weather.
5. If you get stuck, assuming you are wheeling alone, which you shouldn't be doing anyway, try to reach somebody by Cell, CB, Ham whatever you have.
6. Stay warm and protected from the environment, consider where you are and think hard before you walk out. If the walk is far, it's usually better to stay put.
I disagree with this.....
In rain, water and lightning are your #1 worries. Freezing to death is less of a chance unless it's an ice storm.
I disagree with this.
I'd be willing to bet more people die of hypothermia in summer than winter.
A sudden mountain thunder storm can soak you. If your wearing cotton, say jeans and a tee shirt, and an hour hike from you car you are in real danger of death.
A simple contractors trash bag in your day pack will make a poncho to keep you dry.
What about getting stranded, getting stuck so bad you (or you and a friend) can't get it recovered?