The honest answer is to find an injector that knows facial anatomy well and can avoid injecting into small blood vessels. Taking aspirin before the treatment may increase the risk of a bruise but if you take aspirin for a medical reason please do not stop it without medical advice.
How to treat a bruise
If a small bruise is caused after treatment we recommend using ice on the area to reduce the bruise. Be careful to wrap the ice in a towel/place it inside a rubber glove to reduce the risk of a cold burn on the skin.