What is the best way to keep a penis hard after erection?

The only ways to keep the penis erect are continued stimulation and medication. Stimulation can be in the form of arousal (looking at or thinking of arousing things) or through directly touching or manipulating the penis.

One caution is that it is not good for the penis to be erect for over 4 hours. You do not want to compromise the blood flow and end up losing your penis to gangrene and subsequent amputation.
The only ways to keep the penis erect are continued stimulation and medication. Stimulation can be in the form of arousal (looking at or thinking of arousing things) or through directly touching or manipulating the penis.

One caution is that it is not good for the penis to be erect for over 4 hours. You do not want to compromise the blood flow and end up losing your penis to gangrene and subsequent amputation.