Speaking from experience with this pregnancy... and I am certainly not trying to be scary... just putting this out there, because I posted basically the same post as you about a month ago.
You can certainly drink lots of water, with some salt, like the PPs said... and increase your alternate fluid intake as well (watermelons, cucumbers). You can also get in a pool/tub - submerge your body... which can help push excess fluid from your skin into your body.
Realize that your fluid levels are mostly a reflection on your placenta's function, and your baby's ability to pee (in simple terms, haha). If your placenta can't process the fluid, your baby gets less, which means he won't be able to pee enough... and as your fluid levels get lower, baby has less ability to move into position, and the chances of his cord getting compressed increase.
In my experience this prenancy, no matter how much fluid I drank, my fluid levels continued to drop. It turns out that I had calcifications of my placenta - which are normal on due or overdue pregnancies (when examined post partum), but can indicate a problem earlier on.
I did end up delivering early (36 weeks) because my fluid levels had dropped to 2, and DS' heartbeat was dipping on every contraction (even BH) I was having.
Of course, it is normal for fluid levels to decrease gradually as pregnancy progresses... what number were they getting on the ultrasound? At 39 weeks, I don't think you need to be too worried, unless your levels are really low. Is your baby moving normally?