What I used to do was kegel and try to hold it but then reverse kegel or just reverse kegel when about to cum, I usually can stay hard and go 2 or 3 more times.
I'm not sure if it works 100% as I'm still learning how to do it.
But lately I've been able to go like 5-6 times and still feel horny as hell, and I'd have to keep wacking or fucking until I can't get hard any more. I've also been able to shoot as much cum as peter north. (I loved that era) I have like a load of sex books that I'm going to start reading that way when I get my 9x6 I'll be a sex god to go with such a huge rod.