Can you take a diuretic prior to getting facial fillers? Does it help reduce the chance of getting water bags under your eyes?

A diuretic is highly unlikely to make any significant change in the swelling or bruising you will have due to an injection. Restylane naturally takes on water. You can avoid sodium, sleep with your head slightly elevated and apply cold compresses for 10 mins several times a day. Also, avoid ASA products, ibuprofen and alcohol prior to injections to reduce chances of swelling and bruising. All the Best. 

Swelling and bruising are expected side-effects following injections under the skin. Diuretics or water pills will not reduce or prevent this swelling from occurring.Due to the attraction of water to most fillers creating the volumizing effect you desire will also develop the water bags under the eyes you describe. A Board Certified Plastic Surgeon or other well experienced expert injectors who match the appropriate filler and understands the anatomy of the facial region with skillful techniques will provide the best enhancements you seek. I have found that:ice compresses before and after the procedure until you go to sleep,non-sedating anti-histamines such as loratidine or certidine, and fresh pineapple eaten the day before, of the procedure and the day after will more rapidly reduce swelling and bruising.I hope this is helpful. All the best!

A diuretic is safe for use while getting Restylane or Juvederm, but I would not expect it to reduce the chance of side-effects or complications.  More important, is the technique and skill of the injector.  While bruising is quite common when injecting around the eyes, I haven't had a person complain of "water bags" or even significant swelling after treatment.