Hi, I was wondering what can I do to keep a harder errection for a longer time without the use of perscription drugs.

Don't masturbate too. Often; increase the blood flow by breathing deeply; don't stress yourself out about how long you'll stay hard; talk about your fantasies; stop worrying about the last time you had sex and it did not go your way; don't focus your mind only on penetration; take your time in lovemaking. Don't forget that the biggest sex organ is the brain.