How Can I Prevent Bruising from Restylane or Juvederm?

Bruising can occur from any injection but there are steps you can take to help minimize the chances.  Applying ice or cold compress to the area immediately prior to injection and intermittently after injection may help.  Avoid products that can increase bleeding such as aspirin, ibuprofen, and Vitamin E a week prior to injection is also recommended.

What you do before your injections can be more important than what you do after.With more doctors using cannulas, bruising is less of an issue since you'll require fewer needle sticks. Also, the use of a vein detection device is also a great help.Blood thinners and "heart healthy" supplements can increase bruising. Always tell your dermatologist or surgeon if you are taking RX blood thinners, aspirin, ibuprofen, fish oil, etc.Post treatment icing when instructed by your physician can be helpful, but don't overdo it and don't put ice cubes directly on your skin. Too much cold too long can damage your skin. Many physicians use balloons filled with cold water.

Thank you for the question.  There are steps that can be taken to reduce the swelling.  
Before the treatment, it is advised that you avoid medications and substances that increase the risk of bruising.  The patient may be asked not to take aspirin or NSAIDs for at least 7 days, unless discontinuation of these poses a significant risk.  Herbal medications (such as St John’s wort, ginkgo biloba, vitamin E, garlic, ginseng, fish oil) that predispose to bleeding and bruising should be withheld.  Preoperative ingestion of some homeopathic medications such as arnica may reduce bruising, although further investigations are needed to confirm this.
It is also imperitive that you choose an experienced injector who uses a technique to minimize bruising.  This is an operator dependent procedure and so you need to be in safe hands.
After the procedure I also advise my patients to ice the area for up to 10 minutes every hour.  Reusable ice packs can be used or a bag of frozen peas can be used to mold over the area.  A short course of steroid can also accelerate the healing process.  Anectotally, Arnica forte can be used to prevent bruising, but no definitive evidence exists to suggest that is always successful. 
If you would like to learn more about injectable fillers with Restylane and Juvederm I recommend an in person consultation with a board certified dermatologist with cosmetic training.