for long time sex

Wear a condom.  Don't get a cheap brand, but something like a basic Trojan condom should work.

Masturbate once or twice before you sex.  Every orgasm takes a bit longer than the last one.

Think about other things, if you can.  Think about work, or school, or anything other than the person you're with at the moment.

Have sex more often, you'll build up your stamina.

Gain some weight.  Science: People that are a little overweight take longer to orgasm, and have sometimes been called better lovers, overall.