I hear what some of the replies to the OP are saying about you woman not caring how quick you are as long as she orgasms, and I'm sure there is truth to that. And I get the guy asking why you want masturbation to last, but I totally understand why you want to. In fact, practicing lasting during masturbation is a good strategy for building up endurance. I know that I like to last longer because I'M HAVING A FUCKING GOOD TIME and I don't want it to be over yet.
When I was younger maybe I could cum and then go for a second round, but now it's more trouble than it's worth often. So I like to make it last longer building up to my first (and sometimes only) orgasm. I've found a cockring can help, but that's not always available. Thinking about something else doesn't work so much for me. The best thing in my experience is being very "in the moment" so I know when I'm getting close, then slowing it down, changing, stopping to kiss some more, or whatever until the on the edge feeling subsides seems to work the best.
Mainly, you need to be very in tune with the difference between close, almost there, and over the edge. If you're like me, that lines not always consistent.
One last thing, for me at least. If I've not jacked off or had sex of any kind for awhile, I often shoot much sooner than I want. So, regular sex (by yourself or with a partner) can do wonders for your stamina.