Anytime a needle is placed through the skin, the risk of bruising exists. In my practice, I do several things to help reduce this risk when injecting fillers (juvederm, restylane, etc) or Botox. While some of this can be overkill, it at least gives my patients some options to help minimize their chances of having a bruise:
1) I ask patients to not take aspirin or ibuprofen for a few days before and after the procedure
2) No heavy exercise for 24 hrs after the procedure (when you really get your heart pumping and the blood pressure up, it can increase the chance of a small bruise)
3) For patients who tell me they bruise easily, I also ask them to refrain from alcohol for a day or so around the procedure. In addition, certain herbal meds can increase the chance of bruising as well, so those can be stopped before the procedure.
Finally, and most importantly I think, it does come down to the technique of the surgeon injecting, and experienced injectors in general have a very low bruising rate. Most of us will have our patients ice before and after the procedure, and ask them to ice for several hours afterwards as well if possible. Despite this, there is always a risk, so I tell my patients to be prepared for this, and we can usually use simple makeup to cover the area while the bruising subsides.