What causes leg cramps and how do i prevent them?

i have been having leg cramps a lot, which i understand is normal at this stage of pregnancy. but no matter how i place my legs (bent or extended, overlapping so they are not on top of each other), they ALWAYS cramp! i know tonic water helps, but i want to prevent it so i do not have to get up or toss and turn. btw i am 38 weeks.