To add another underlying cause – For anyone that is a bad asthmatic (i.e. takes steroid medication daily), one of the side effects of this is bruising extremely easily, and I suffered badly from this the first 12 months. I will second that icing and arnica help with the bruising!
After speaking to someone trained in Eastern Medicine, they suggested taking Calcium with D3, and this did help a couple of the girls who had no other reason for bruising a lot.
Theory: As calcium is important in cell membrane integrity, if you do not have enough your capillaries can become slightly more brittle. D3 helps your body absorb the calcium. Also, trying to improve your circulation will aid this.
Hope this helps! x