What can a man do in order to improve his erection?

Many factors could potentially influence one’s sexual arousal at a certain moment. Beyond biological factors, such as the penile vessels, the hormones etc, there are numerous factors related to the relationship, the daily mood, the daily life events etc that could have an indirect influence on arousal. For example, on a day with too much work stress, one might experience a difficulty to achieve an adequate erection. But the effect of work stress on erectile function is probably indirect. Thus it may not be the work stress per se, but rather the fact that because of work stress one felt a reduced desire to have sex or was easily distracted away from the sexual stimulation. Too make things easier; I have selected five factors that to my experience are the most crucial factors that influence sexual arousal, beyond biological parameters.  If you take care of these, then arousal will be enhanced!