Fortunately, most dehydrated Hot Yoga students only experience the first column.
Focus on hydration before you come to class. Drink at least 24-32 oz of water before you even get there
Electrolytes really make a difference. You can buy them at any health food store, organic market or at many yoga studios. My favorite is Ultima. Carry them with you and put them in the water you drink prior to, during and after practice.
Sip water during practice.
Many yogi's drink coffee prior to practice but do know that caffeine can dehydrate the cells. If you drink coffee, you need even more water.
If you start to get overheated, don't keep pushing. Take rest, modify the practice, move your mat, ask for fans...whatever you have to do to get cooler
Eat foods with higher water content like fruits and vegetables. The water keeps you hydrated and you feel lighter when you practice.
If you feel moderately dehydrated when you leave
Drink 24-36 oz of electrolytes and water
Get into a cool area
Dehydration headaches are often accompanied by nausea. If you feel a headache coming on, take your over the counter pain med of choice before it gets bad
Add more water to your diet. You obviously didn't get in enough before coming.
I am not a doctor. This is just what works for me. Take this advice at your own risk.
If you love hot yoga, what are your favorite dehydration tools? (if you don't love hot yoga, please don't respond with your treatise against the practice)