I recently reviewed an article in Dermatologic Surgery (2008 June; 34:S105) on the complication rates of Restylane and Perlane hyaluronic acid fillers used in the nasolabial folds and the angles of the mouth.  The investigators are well known to me (Richard Glogau in particular) and I respect their data.  What they found is that in their hands, the risk of bruising, swelling, and tenderness immediately after treatment with these agents is directly related to rapid injection, overcorrection and an injection technique called fanning.  In fanning, a single injection point is used with the needle advanced in multiple directions like a fan.  Anyway, I have found the same results in my patients and was pleased to see this data published.  Injection technique is the key to success, and if you are contemplating a dermal filler be sure and mention these items to your provider!