One way to stay super hard and for longer is to use a heavy wide rubber band like the type used on newspapers (too narrow will break or hurt) to wrap in a figure 8 around the base of your cock and then scrotum after you get an erection. Make sure it is tight enough to feel it but not too tight to unduly cut off blood circulation.
It then feels amazing if you stroke off or are being stroked off. It prolongs the length and intensity of your orgasm. You also spew cum like crazy.
My girlfriend likes to jerk me off in semi public places like the strip bar she works at and this works wonders everytime! I carry spares in my pocket when I'm out just so she can play or I can show off-ha! Give it a try and let me know what you think.