Move slowly. Don't get up, bend over, stand up to fast.
Definitely try and drink more water. With all the increased blood volume, your body actually needs about a gallon of water a day! I'm 30 weeks and not the best about drinking tons of water- and I know the days I don't drink enough I feel light headed and dizzy. The extra water will help keep your blood pressure up and keep you from blacking out :)
I had that, it was low iron, and probably my low blood pressure added to it.. I've been on iron supplements for probably 5 weeks and it hasn't happened lately except in the shower (which happens pre pregnancy for me anyway). Definitely can't hurt to get your levels of everything checked...
31 weeks today I know exactly how you feel. I normally black out at work (retail) if I get too hot or we get busy. My pulse rate jumps up really high. Have your doctor check your iron, my dizziness was due to iron deficiency. Some days it's hard to even get out of bed my head is so funny feeling, but I was told it's normal. Also keep check on your blood pressure.